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ngc3314
2011-Sep-14, 04:08 PM
Mike Disney has worried about selection effects in detection of galaxies (and their results on what we know about the galaxy population) for about 35 years. Here's his latest, no doubt deliberately provocative but a set of conclusions to be seriously wrestled with:

The galaxy ancestor problem (http://arxiv.org/abs/1109.2870), Mike Disney and Huw Lang:


HST finds galaxies whose Tolman dimming should exceed 10 mag. Could evolution alone explain these as our ancestor galaxies? Or could they be representatives of quite a different dynasty whose descendants are no longer prominent today? We explore this latter hypothesis and argue that Surface Brightness Selection Effects naturally bring into focus quite different dynasties from different redshifts. Thus the HST z=7 galaxies could be examples of galaxies whose descendants are both too small and too choked with dust to be recognizable in our neighborhood easily today. Conversely the ancestors of the Milky Way and its obvious neighbors will have completely sunk below the sky at z>1.2 although their diffuse light could account for the missing Reionization flux. This Succeeding Prominent Dynasties Hypothesis (SPDH) fits the existing observations both naturally and well,including the bizarre distributions of galaxy surface brightnesses found in deep fields, the angular size ~ inverse (1+z) law,'Downsizing' which turns out to be an 'illusion' in the sense that it does not imply evolution, 'Infant Mortality', i.e. the discrepancy between stars born and stars seen, and finally the recently discovered and unexpected excess of QSOAL DLAs at high redshift. If the SPDH is true then a large proportion of galaxies remain sunk from sight, probably at all redshifts. We show that fishing them out of the sky by their optical emissions alone will be practically impossible, even when they are nearby. More ingenious methods will be needed to detect them. It follows that disentangling galaxy evolution through studying ever higher redshift galaxies may be a forlorn hope because one will be comparing young apples with old oranges, not descendants with their own ancestors.


There are a lot of wrinkles I plan to ping him about - gains from the very blue colors of young stellar populations, detection of galaxies from their UV-bright compact regions at substantial redshift, dusty-galaxy detections in the FIR and submm when we have accurate enough coordinates - but these visibility limits are important issues. In the most extreme cases they consider, much of what we think we know about galaxy evolution is instead a reflection of selection effects operating on a galaxy population much broader than we can easily detect at any single redshift.

kzb
2011-Sep-14, 05:01 PM
Thanks for posting this, looks very interesting. Not read it all yet, and I won't understand it all when I have, but look at this quote:

.....high redshift galaxies are truly bizarre.
They are one or two orders of magnitude smaller in physical size, while their intrinsic surface
brightnesss must be 9 mag or 4000 times higher. Moreover, and this is even more extraordi-
nary, they must have systematically adjusted their sizes and their SBs [surface brightness] over cosmic time...

Jerry
2011-Sep-15, 11:25 AM
Thanks for posting this, looks very interesting. Not read it all yet, and I won't understand it all when I have, but look at this quote:

.....high redshift galaxies are truly bizarre.
They are one or two orders of magnitude smaller in physical size, while their intrinsic surface
brightnesss must be 9 mag or 4000 times higher. Moreover, and this is even more extraordi-
nary, they must have systematically adjusted their sizes and their SBs [surface brightness] over cosmic time...

Truly bizarre, or once again, as Disney suggests, are selection effects playing havoc with our perceptions. Are there galaxies that suffer from lens pinching? Could there be a condensation of the light path so that the size is artificially small and the magnitude artificially bright?

KABOOM
2011-Sep-15, 01:13 PM
The linked paper was fairly dense (at least to me).

If anyone can provide more of a layman's explanation to the concept of galaxies such as the MW or Andromeda, "falling off the horizon" due to low redshift scores, I would appreciate it. On the surface it made it sound like there could be a multitude of Local Cluster type galaxies much closer than other outlies within our observable horizon that are simply no longer visible. So where exactly are they? Simply out there but luminouse enough to be detected with our current tecnhology?

ngc3314
2011-Sep-15, 02:40 PM
There are several factors at play in the detectability of galaxies. One is observed surface brightness - energy received per square arcsecond, or example. As this drops below the interfering level of sky brightness (airglow, light pollution, starlight scattered from interstellar dust) the required exposure times for detection grow rapidly for fainter levels. This sets a fairly hard limit on what galaxies we can detect in a particular regime - better from space, and better at wavelengths with the lowest levels of interfering diffuse light.

Another major effect is the Tolman dimming. Cosmological pictures with expanding spacetime (or any coordinate equivalent) make a robust prediction that the surface brightness of otherwise identical objects, measured in the same way, drops off with redshift z according to (1+z)-4. This is what would make Milky Way analogs "drop out" at higher redshifts, although the details depend on how we observe them - higher resolution lets us see the bright cores and star-forming regions, while redshifting ultraviolet into the visible band would exacerbate light loss due to dust while presenting us brighter and more compact star-forming regions when they are not heavily dust-reddened. One clear result of the Tolman dimming is that our optcally-chosen galaxy samples at high redshifts cannot fail to be biased in favor of galaxies or pieces of galaxies which are blue (UV-bright) of unusually high surface brightness.

Their visibility selection function for galaxies includes a third factor - sufficiently small galaxies, which have the highest surface brightness, will not be selected by typical surveys and will be identified only spectroscopically or when their colors are uifficiently different from stars.

None of the foregoing is at all controversial, this is standard stuff. Where these authors set off fireworks is in the claim that these effects limit our view of the galaxy population so strongly that most of what is usually interpreted as evolution in the galaxy population instead results from redshift-dependent selection of galaxies from a wider range of properties than normally assumed, so strongly that our empirical constrains on galaxy evolution from multi-redshift comparison weakens dramatically. There are a lot of details to pry into to see how strong an effect this is. For example, H I wide-field surveys turn up very few "dark" galaxies where we see the gas but not starlight - but how true this is depends on how precisely we can associate the 21-cm objects with detected galaxies. And as above, we could still pick up star-forming regions in a galaxy which has otherwise faded past our detection limits, and very dusty galaxies below our optical threshold should show up as far-infrared sources (as indeed some do).

Nereid
2011-Sep-15, 05:03 PM
Surely wide surveys like WISE and UKIDSS (http://www.ukidss.org/), and narrow ones like the various multi-wavelength/band observation campaigns of Chandra-S etc, provide a very good handle on this sort of thing?

In any case, doesn't it come down, in the end, to the extent to which various hypotheses are consistent with all the relevant observational data?

I must be missing something ...

kzb
2011-Sep-15, 05:28 PM
Another point from the paper is that before Hubble was launched, they assumed that redshift z>2 galaxies could not be observed, precisely because of this Tolman dimming. But "that got forgotten about" and they are observing galaxies now at redshift 7 and beyond.

I've a question though: current galaxies could have formed from mergers of smaller galaxies. So could the current crop of large but low surface brightness galaxies not have formed from mergers of the small but high SB galaxies?

ngc3314
2011-Sep-15, 05:36 PM
Their claim (which I'm still looking at in the context of details listed above) is that the redshift/surface brightness selection is so strong that it may dominate over actual evolutionary effects. For example, Lyman-break galaxies at z~3 will have surface brightness dimmed by 44=256 times, so in order to be as bright as we see, they must have prodigious (UV) surface brightness which has virtually no local analogs among entire galaxies. Elliptical galaxies fare a bit better observationally because of their light concentration. The authors like to point out (in Disney's case, not for the first time) that the range of average surface brightness of catalogued galaxies (by various standard measures - within the half-light radius, Petrosian radius, isophotal radius which changes with z-dimming) is much narrower than the amount of Tolman dimming expected to significant redshifts, so it would be a remarkable coincidence of the mean star-forming rate just managed to cancel it out. This looks like much more of a problem in a global view than when looking at one particular class of galaxy (although they further claim that we could be getting it wrong when trying to trace a single galaxy type across wide spans on redshift).

On the other hand, the maximum surface brightness of starburst regions in galaxies is pretty constant with z when taking Tolman dimming into account, although the linear scale and thus luminosity changes a lot. And it is clear that there is no significantly numerous local population of galaxies so compact that they masquerade as stars in, or example, the SDSS (all those words are significant) - they claim that these would be the descendants of galaxies easy to see in HST images at high z, unless they are now so dusty as to be mostly far-infrared sources..

kzb
2011-Sep-16, 03:12 PM
The other alternative of course is there is something wrong about universal expansion/redshift theories.

Nereid
2011-Sep-17, 08:17 AM
The other alternative of course is there is something wrong about universal expansion/redshift theories.
Never say never, of course, but no, that's not really a viable alternative.

Galaxies are really complicated things, and how they evolve (change over time) doubly so. For example, not that long ago it was widely accepted that they mostly just evolved in isolation; now I think it's fair to say that very few galaxies we see today, locally, got to be the way they are/seem without all sorts of influences from their environment (including 'major mergers').

The alternative you mention would involve 'unexplaining' (and so re-explaining) a staggering number of observations, of many different kinds.

George
2011-Sep-17, 07:49 PM
Another major effect is the Tolman dimming. Cosmological pictures with expanding spacetime (or any coordinate equivalent) make a robust prediction that the surface brightness of otherwise identical objects, measured in the same way, drops off with redshift z according to (1+z)-4.
Wasn't this tweaked from a 4th power to something between 2.6 to 3.4? Lobin & Sandage, 2001 (The Effect of the Point-Spread Function and Galaxy Ellipticity )

[I'm just drilling a hole in the wall here so I can watch. :)

Nereid
2011-Sep-17, 10:49 PM
I've just started to read the preprint*, and I figure it'd take me a solid month of work to really understand it. And it would, indeed, take me that much effort, how many of BAUT's regulars would be able to 'get it' significantly faster? ngc3314, obviously, and perhaps a dozen others?

Anyway, here are some immediate thoughts (mostly questions):

-> "objects with an exponentially declining light distribution (virtually all galaxies bar Giant Ellipticals; see later)": we've now 'seen' galaxies in wavebands from GeV gammas to MHz radio; in what slices of this enormous range are virtually all (local) galaxies exponential?

-> are all quasars (BL Lac objects, etc) the nuclei of galaxies?

-> what happens to the Visibility window when a (distant) galaxy is (strongly, gravitationally) lensed?

-> for the Hubble, the 'sky' is very dark, and 'size visibility' sharp. How does Hubble's P and FWHM compare with those of a small, ground-based telescope (say, 0.4m) in a heavily light-polluted location (around full Moon perhaps)?

-> some galaxies emit a great deal of their visual waveband light in the form of emission lines, e.g. H-alpha, [OIII]. Are such galaxies exponential when observed in narrow bands around these lines?

* it certainly is a preprint! There are a number of things - all minor? - that need to be fixed before it's ready for publication

Nereid
2011-Sep-18, 01:34 AM
OK, maybe not a month, but at least a week! :razz:

Having now read the whole paper, some immediate impressions:

* it certainly is a preprint! In the Acknowledgements section: "Nino Disney, Richard Elliott, and Joe Romano (U Texas, Brownsville) for indispensable help with word processing": well, there are plenty of typos (etc) that need fixing

* of the points in my last post, the preprint deals explicitly (and at some length) with the 4th ("for the Hubble, the 'sky' is very dark, ...")

* some of the predictions of the SPD hypothesis (as presented in this preprint) would seem to be fairly easily tested, doing nothing more than some analyses of data from readily available astronomical datasets

* general question: what's the current state-of-art re spectroscopic redshift estimations in the mid-IR and far-IR?

Nereid
2011-Sep-18, 03:47 PM
I've now got quite a few ideas of some research into the things Disney and Lang cover in their preprint. Unfortunately, I can do little, if any, of this research entirely on my own. So, is anyone reading this interested in collaborating?

(you can send me a PM if you don't want to write a post)

Jerry
2011-Sep-19, 01:35 AM
Never say never, of course, but no, that's not really a viable alternative.

Galaxies are really complicated things, and how they evolve (change over time) doubly so. For example, not that long ago it was widely accepted that they mostly just evolved in isolation; now I think it's fair to say that very few galaxies we see today, locally, got to be the way they are/seem without all sorts of influences from their environment (including 'major mergers').

The alternative you mention would involve 'unexplaining' (and so re-explaining) a staggering number of observations, of many different kinds.

Ya, a lot more than a small group of researchers could sort out.

The same argument, by the by, has been made concerning the 'staggering number' of other observations that must be wrong if the 'dark energy' solution is not the correct answer to the observed brightness of distant supernovae.

Galaxy brightness and supernove magnitude determinations should more-or-less trump a staggering number of lesser approximations based upon weaker assumptions.

StupendousMan
2011-Sep-19, 12:25 PM
-> "objects with an exponentially declining light distribution (virtually all galaxies bar Giant Ellipticals; see later)": we've now 'seen' galaxies in wavebands from GeV gammas to MHz radio; in what slices of this enormous range are virtually all (local) galaxies exponential?


The statement refers to visible light, and to the near-IR to a lesser extent. At high energies, emission from galaxies is dominated by a relatively few discrete sources, not the billions of stars which produce the visible and near-IR.



-> are all quasars (BL Lac objects, etc) the nuclei of galaxies?


Yes. Some people are searching for cases in which a supermassive black hole has left its home galaxy, due to the gravitational influence of another supermassive black hole, but they haven't found many yet.



-> what happens to the Visibility window when a (distant) galaxy is (strongly, gravitationally) lensed?


That galaxy becomes visible at higher redshifts than it would be otherwise. This isn't very common, by the way, so it won't have any major effect on the statistics of large samples.



-> for the Hubble, the 'sky' is very dark, and 'size visibility' sharp. How does Hubble's P and FWHM compare with those of a small, ground-based telescope (say, 0.4m) in a heavily light-polluted location (around full Moon perhaps)?


HST's FWHM can be approximated by the simple diffraction limit: wavelength / diameter of telescope. So, in the optical, 500 nm / 2.4 m = 2.1 x 10^(-7) radians = 0.04 arcsec. Small ground-based telescopes without active optics will have a FWHM of order 1 arcsec, due to the atmosphere.

The sky brightness from the optical depends on location and wavelength. Choose B-band, for an example. The B-band sky brightness at the best mountain-top sites is of order 22 magnitudes per square arcsecond. The B-band sky brightness at poor sites is much higher -- perhaps mag 17 mag per square arcsecond, and worse if the full moon is present. The difference is much larger if one moves to the near-IR, where HST's sky background is much, much, MUCH lower than the sky background from any ground-based site.

Are you trying to compute some sort of detection limit between these two cases?




-> some galaxies emit a great deal of their visual waveband light in the form of emission lines, e.g. H-alpha, [OIII]. Are such galaxies exponential when observed in narrow bands around these lines?


No, in the narrow bands, one will see chunky bits of emission scattered around the extent of the galaxy. After all, the emission comes from HII regions, which occur only in the neighborhood of hot young stars (yes, and a tiny bit from planetary nebulae, but they are discrete little sources, too).

Nereid
2011-Sep-19, 01:08 PM
Ya, a lot more than a small group of researchers could sort out.

The same argument, by the by, has been made concerning the 'staggering number' of other observations that must be wrong if the 'dark energy' solution is not the correct answer to the observed brightness of distant supernovae.
Can we stick with the topic, please?


Galaxy brightness and supernove magnitude determinations should more-or-less trump a staggering number of lesser approximations based upon weaker assumptions.
I guess, from this comment, it would be fair to say that you didn't actually read the Disney and Lang preprint? Or, that you read it, but didn't understand it?

Nereid
2011-Sep-19, 01:24 PM
Thanks StupendousMan! :)

The statement refers to visible light, and to the near-IR to a lesser extent. At high energies, emission from galaxies is dominated by a relatively few discrete sources, not the billions of stars which produce the visible and near-IR.
If so, then there's a major piece missing in the Disney and Lang presentation*: are galaxies overwhelmingly exponential in the UV, from the Lyman limit to ~300 nm? If not, then most of the case they make is irrelevant (or at least needs caveats/further work).


Yes. Some people are searching for cases in which a supermassive black hole has left its home galaxy, due to the gravitational influence of another supermassive black hole, but they haven't found many yet.
There are a lot of papers on the selection effects at work wrt quasars. However, if there's good reason to be confident we have a handle on their co-moving space density, as a function of redshift, then we already know something about the space density of at least some galaxies (i.e. the hosts of the quasars). At least in principle, that allows the opportunity to test some aspects of the Disney&Lang hypothesis (or hypotheses).


That galaxy becomes visible at higher redshifts than it would be otherwise. This isn't very common, by the way, so it won't have any major effect on the statistics of large samples.
Sure.

Assuming that the object (galaxy, cluster) doing the lensing is not associated in any way with the object lensed (other than by serving as the lens!), then the distribution of (leased) galaxy properties (SB profile, size, etc) should provide a test of the Disney&Lang hypothesis (or hypotheses).


HST's FWHM can be approximated by the simple diffraction limit: wavelength / diameter of telescope. So, in the optical, 500 nm / 2.4 m = 2.1 x 10^(-7) radians = 0.04 arcsec. Small ground-based telescopes without active optics will have a FWHM of order 1 arcsec, due to the atmosphere.

The sky brightness from the optical depends on location and wavelength. Choose B-band, for an example. The B-band sky brightness at the best mountain-top sites is of order 22 magnitudes per square arcsecond. The B-band sky brightness at poor sites is much higher -- perhaps mag 17 mag per square arcsecond, and worse if the full moon is present. The difference is much larger if one moves to the near-IR, where HST's sky background is much, much, MUCH lower than the sky background from any ground-based site.

Are you trying to compute some sort of detection limit between these two cases?
The question was ill-posed and premature; the Disney&Lang preprint covers what I am interested in (I just hadn't read far enough).


No, in the narrow bands, one will see chunky bits of emission scattered around the extent of the galaxy. After all, the emission comes from HII regions, which occur only in the neighborhood of hot young stars (yes, and a tiny bit from planetary nebulae, but they are discrete little sources, too).
And that too suggests several possible tests of the Disney&Lang hypothesis (or hypotheses). In fact, they actually mention this indirectly, when they talk about HI surveys.

* It may, perhaps, be there ... and I simply didn't read (or absorb) it

Jerry
2011-Sep-19, 02:22 PM
Can we stick with the topic, please?
which topic? Disney is proposing a convoluted solution to an evolution-induced problem: We can't find the progenitors of todays galaxies, and we can't find the children of yesterday's galaxies. Isolating a single, or even a pair of cosmic uncertainties while declaring that the bulk of the evidence as sound is poor justification for introducing new, sliding parameters to quench the probem. If, for example, increasingly dusty environments are quenching the surface brightness of AGN galaxies, we should see an increase in the dust-induced redness of this population that follows roughly the same inverse power rule as the AGN magnitude creep. I don't think that the evidence in hand suggests this.


I guess, from this comment, it would be fair to say that you didn't actually read the Disney and Lang preprint? Or, that you read it, but didn't understand it?
Perused, and jumped to the usual conclusion: The Tolman law, the theory that leads to the conclusion that surface brightness has evolved, should be suspected. Rather than resolving every one of these reoccurring problems by pounding every observation that is at odds with current theoretical expectations into submission we should look harder for other explanations for the discrepancy. The irony here is that I agree with Disney: Selection effects are masking our ability to trace galactic history. I would simply add that our obsession with mechanics developed in the early twentieth century hamstring theoretical development, too. If there is no obvious evolutionary trail from the past universe to the present; it may be the evolutionary theory that needs to be discarded rather than salting the theory with dark stuff to taste.

A LOT of the probems Disney is dealing with evaporate if the Tolman z^-4 dimming rule is discarded.

Nereid
2011-Sep-19, 02:57 PM
which topic? Disney is proposing a convoluted solution to an evolution-induced problem: We can't find the progenitors of todays galaxies, and we can't find the children of yesterday's galaxies. Isolating a single, or even a pair of cosmic uncertainties while declaring that the bulk of the evidence as sound is poor justification for introducing new, sliding parameters to quench the probem.
The usual Jerry interpretation, eh?

It'd be nice to once, just once, engage in a meaningful discussion with you Jerry, on topics like this.


If, for example, increasingly dusty environments are quenching the surface brightness of AGN galaxies, we should see an increase in the dust-induced redness of this population that follows roughly the same inverse power rule as the AGN magnitude creep. I don't think that the evidence in hand suggests this.
Perhaps it is, perhaps it isn't.

However, as usual, it would seem that you and I have read completely different documents!

For example, it would seem that you found nothing the least bit unusual, suspicious, questionable, assumptions-too-far, etc, etc, etc in the Disney&Lang preprint; rather, you bought the most speculative parts of their discussion section at face value.

Doesn't it ever bother you that you approach this sort of material in such a blatantly biased way?


Perused, and jumped to the usual conclusion: The Tolman law, the theory that leads to the conclusion that surface brightness has evolved, should be suspected. Rather than resolving every one of these reoccurring problems by pounding every observation that is at odds with current theoretical expectations into submission we should look harder for other explanations for the discrepancy.
Thanks, you just made my point, in spades.

Straight question Jerry: did you find anything, anything at all, in the Disney&Lang preprint that might give rise to even a niggle of doubt, in the mind of a 'cosmology skeptic' like you?


The irony here is that I agree with Disney: Selection effects are masking our ability to trace galactic history.
Don't you think it just the teensiest bit ironical that, based on what you've posted so far in this thread, it seems you haven't actually read the Disney&Lang preprint?


I would simply add that our obsession with mechanics developed in the early twentieth century hamstring theoretical development, too. If there is no obvious evolutionary trail from the past universe to the present; it may be the evolutionary theory that needs to be discarded rather than salting the theory with dark stuff to taste.
It's easy to spin an enjoyable yarn about this sort of stuff, isn't it?

Is it too much to ask this: when do you expect to roll up your sleeves and do the hard yakka of putting your nice words to the rack of quantitative analysis?

Tensor
2011-Sep-19, 04:26 PM
A LOT of the probems Disney is dealing with evaporate if the Tolman z^-4 dimming rule is discarded.

Can you show which problems, and by how much those problems go away if the Tolman dimming rule is discarded? Or,as is your history, is this just another throwaway claim that you have no intention of providing any kind of quantitative support for?

ngc3314
2011-Sep-19, 04:33 PM
I think this issue has come up before in the SN Ia context - an unresolved source displays two factors of (1+z) of the Tolman dimming (we don't see the other two unless we spatially resolve it since they come from angular size). To the range where SN Ia are detected (z~1.3), that part of the effect is about an order of magnitude greater than the deviations interpreted as due to acceleration. So getting rid of the expansion-attributed effects makes the SN data much, much worse instead of getting rid of a problem. One has to look carefully at a particular study's way of doing the K-correction to see where this is done, BTW - these are heuristically the photon-energy and arrival-time-dilation terms; bandwidth narrowing and band shifting are different beasts best done from a sample spectrum.

peteshimmon
2011-Sep-19, 06:36 PM
This kind of reminds me of Radio Source
counts in the sixties and conclusions
being drawn.

Poor Sir Fred...

Nereid
2011-Sep-24, 11:37 AM
I've now got quite a few ideas of some research into the things Disney and Lang cover in their preprint. Unfortunately, I can do little, if any, of this research entirely on my own. So, is anyone reading this interested in collaborating?

(you can send me a PM if you don't want to write a post)
No one eh?

StupendousMan
2011-Sep-24, 03:45 PM
No one eh?

Fire away, Nereid. An open discussion on this thread might be interesting and educational for many readers.

George
2011-Sep-24, 05:39 PM
Fire away, Nereid. An open discussion on this thread might be interesting and educational for many readers. I'm more the lurker, but this topic looks like we might get some nice juice for the squeeze. Just learning all the differenct effects and pricinciples involved will be worth it for me.

Nereid
2011-Sep-24, 08:22 PM
Fire away, Nereid. An open discussion on this thread might be interesting and educational for many readers.
Excellent point! :clap:

OK, let's start with that fantastic, free, public resource, SDSS. Would it be better to use DR7, or DR8? Why? Myself, I think it depends on what we want to do.

We could start by thinking up tests of the Disney&Lang assumption, that galaxies have exponential surface brightness (SB) profiles (other than giant ellipticals, or BCGs).

For example*:
-> bin galaxies by redshift, Petrosian radius, and dominance of spectrum by emission lines
-> for bins with sufficiently many members, stack
-> extract the stacked radial SB distribution, by band (u, g, r, i, z)

I recall this has already been done, for z ~ 0.2 BCGs; I'll see if I can dig up the paper.

* There's plenty of data; from memory, there are >600,000 objects identified as galaxies in DR7.

Nereid
2011-Sep-25, 02:48 AM
Tal and van Dokkum (2011) (http://arxiv.org/abs/1102.4330): "The faint stellar halos of massive red galaxies from stacks of more than 42000 SDSS LRG images".

parejkoj
2011-Sep-26, 03:23 PM
Would it be better to use DR7, or DR8? Why?

DR8 has photometry over a larger area, with significantly better sky-subtraction than DR7. If you want to investigate surface brightness issues, you definitely would prefer DR8.

Also, there are just under a million unique galaxies with spectra in DR7/8 (no new extra-galactic spectroscopy in DR8), with redshifts up to ~0.6.

Nereid
2011-Sep-26, 04:33 PM
Thanks parejkoj.

GALEX DR6 (http://galex.stsci.edu/GR6/) may be a good place to start to investigate the nature of galaxy surface brightness in two UV wavebands, as a function of redshift.

BigDon
2011-Sep-26, 06:18 PM
I'm more the lurker, but this topic looks like we might get some nice juice for the squeeze. Just learning all the differenct effects and pricinciples involved will be worth it for me.

I whole heartedly agree.


The usual Jerry interpretation, eh? "snip"

:Picks up drink and steps from between Nereid and Jerry:

I've been here for years and realize there's history and all, but dude!

May I recommend decaf?

Nereid
2011-Sep-26, 07:58 PM
I whole heartedly agree.



:Picks up drink and steps from between Nereid and Jerry:

I've been here for years and realize there's history and all, but dude!

May I recommend decaf?
Put yourself in my shoes BigDon*, how would you respond?

* well, figuratively; if I had to guess I'd say mine are rather too small for your feet

Nereid
2011-Sep-27, 08:58 AM
Let's assume, for now, that Tal and van Dokkum (2011)'s Figure 6 is the light profile of a z = 0.34 LRG, in the five SDSS filters. Let's assume the central wavelength of each filter is (in nm):

u 354.3
g 477.0
r 623.1
i 762.5
z 913.4

At what redshift (approximately) would this LRG appear to be too small to be detected as a galaxy, in SDSS? Assume Tolman dimming, a 'minimum catalogue limit' of 1.4", and 'aberration' (as Disney and Lang use the word). This exercise, involving a specific (though somewhat synthetic) galaxy will - I hope! - illustrate the Disney and Lang hypothesis. It should also bring out some of the messy, real-telescopes-etc, details we may have to deal with.

Jerry, George, and BigDon (and any other interested reader lurker): do you think you could sketch the steps you think should be taken to work out an answer?

StupendousMan
2011-Sep-27, 01:25 PM
To answer this question, one would need to know many things.

a) what is the surface brightness limit for galaxies to be detected and classified as galaxies, in each passband?
b) what is the size limit for galaxies to be detected and classified as galaxies, in each passband?
c) what rules are used in the SDSS software to decide than an object is a galaxy? That is, if an object is detected in the g, r, i passbands, but not the u or z passbands, is it classified as a galaxy, or discarded, or what?
d) what is the spectrum of the sample LRG? Is it a constant, or does it vary as a function of position in the galaxy?

With that information, one can, for any given redshift z, compute the apparent size and surface brightness in each passband (redshifting the spectrum and performing synthetic photometry as needed); determine the size and surface brightness of the galaxy in each passband; decide if the software will detect it and classify it as a galaxy in each passband; and determine how the object will appear, if at all, in the SDSS catalogs.

Pretty basic stuff, but it will take some time.

Nereid
2011-Sep-27, 02:12 PM
To answer this question, one would need to know many things.
Yes indeed.

Jerry, George, BigDon (and other lurkers): how complete is StupendousMan's list?

a) what is the surface brightness limit for galaxies to be detected and classified as galaxies, in each passband?
This one is a biggie.

For the purposes of illustrating the test, let's assume:
u 22.12
g 22.60
r 22.29
i 21.85
z 20.32


b) what is the size limit for galaxies to be detected and classified as galaxies, in each passband?
1.4"

c) what rules are used in the SDSS software to decide than an object is a galaxy? That is, if an object is detected in the g, r, i passbands, but not the u or z passbands, is it classified as a galaxy, or discarded, or what?
For simplicity, let's assume that if it's bigger than 1.4" in any passband (filter, waveband, etc), it's a galaxy.


d) what is the spectrum of the sample LRG? Is it a constant, or does it vary as a function of position in the galaxy?
Another biggie.

Tal and van Dokkum (2011) - TvD11 for short - do present results of their stacking, for the colour gradients (i.e. as a function of radius). LRGs (and ellipticals in general) become bluer away from the nucleus; however, the trend is greatest in the first few (tens of) kpc.

Of course, colours are not spectra; for simplicity, let's assume that our LRG is bland; i.e. the spectrum taken at any position is the same, modulo level.


With that information, one can, for any given redshift z, compute the apparent size and surface brightness in each passband (redshifting the spectrum and performing synthetic photometry as needed); determine the size and surface brightness of the galaxy in each passband; decide if the software will detect it and classify it as a galaxy in each passband; and determine how the object will appear, if at all, in the SDSS catalogs.
I agree, except that ... I think there are a few other things we need to take into account (even if only to convince ourselves that they don't matter, wrt the approximate answer I'm looking for for now: this is an exercise largely in getting a feel for what sorts of things we need to consider, and determining the feasibility of doing some real research).

For example: to what extent do we need to make explicit our cosmological assumptions (other than Tolman dimming and 'aberration')?


Pretty basic stuff, but it will take some time.
Yes, so far it's mostly just writing down the steps, making sure we've not missed something important, and cranking up our googling, skimming, and data-extraction skills ...

ngc3314
2011-Sep-27, 06:43 PM
You can picture an object-by-object approach, if you start by having images of a galaxy at a number of wavelengths. In a simple case, where you picked one for which you have a UV image that would redshift to a desired optical band at a redshift of interest, you would calculate the flux your galaxy would have at the new redshift along with the change in its angular size ("aberration" and all), then make a version of that image with appropriate noise. As an example, take a GALEX near-UV image of some galaxy at z~0.05, consider what it would look like at z-0.25 (HDF F30W) or z=0.87 (HDF F450W) at the angular resolution and noise per pixel of the HST HDF (or other) observations. How much of the galaxy if any) passes a desired S/N threshold? This lets you sample behavior at narrow redshift ranges but needing no assumptions about the spectral behavior, since that's all implicit in the images. The point of doing this for local galaxies which have lots of longer-wavelength data is that you then sort of understand where they fall in optical luminosity, Hubble type, concentration index, and so on, so you can ask how the mix of these entering a catalog changes with redshift.

There were cottage industries doing this when good UV images first became available for lots of galaxies (a major program on the Astro-2 mission, for example, even before HST could do it well). Many show up doing an ADS abstract-word search on "morphogical K-correction".

As far as I can tell, the pieces of cosmology that enter are the angular size-redshift relation, and whether time dilation follows (1+z). The latter comes in by increasing the mean time between photon arrivals. Heuristically, one can thin of the Tolman dimming as having one factor of (1+z) for photon energy, one for photon arrival time, and two for increase in angular size of distant objects compared to the Euclidean case (what Disney and co. call aberration). Additional terms may enter observationally but are purely detector-based; for example, the width of a filter band in the emitted frame decreases as 1/(1+z), but you could in principle avoid that by using a new filter which matches some standard one when used at a particular redshift. (If you know the spectrum, this can be corrected in a purely numerical way).

BigDon
2011-Sep-27, 09:00 PM
Nereid,

Due to time constraints, my BAUT cruising time is up and just finding out about the pop quiz, I beg permission to take up the task of rereading this thread in even greater depth, i.e. follow a link or two, tomorrow morning. I'll try to get back with some sort of answers tomorrow evening Pacific Time, even if it's "I don't know" or "I need more time to rig the mental scaffolding".

Right now a 55gal tank full of weird and brilliant guppys demands my attention. Plus I have to figure out a way to keep possums out of my carnivorous plant collection. The possums drink the standing water they have to have in the basins and knock eveything over. Wait a sec! It just came to me as I was typing this out. I'll try setting aside a big bowl of water like a dog dish! That way they won't have to knock my plants over to drink!

Tomorrow evening then.

George
2011-Sep-28, 02:05 AM
Jerry, George, BigDon (and other lurkers): how complete is StupendousMan's list? All important items, no doubt. What about extinctions, since SDSS is terrestrial, or are the results already atmospherically compensated?

I feel like a kid going to the zoo for the first time. The nuances of distant galaxy exploring is new to me.

In earlier post, I did offer a correction (Sandage) to the Tollman inverse 4th power term, assuming I'm even on the same page of what is being addressed. Was I wrong? Won't the equation be important to resolving some of the questions?

Other questions I have are crude:

Wouldn't heavier intergalactic neutral hydrogen be more prevalent in earlier periods disrupting the Lyman break observations?

Here's a wild one -- Has anyone produced a SED of a region of "empty" space to see the "color of the sky"? A great deal of scattering takes place, which might help reveal more of the light sources. [Is my heliochromology showing? :)]

What of metalicity of early galaxies as it relates to anticipated spectrums?

[I'm reluctant to ask questions that might slow the pace.]



For example: to what extent do we need to make explicit our cosmological assumptions (other than Tolman dimming and 'aberration')? Yes, what are the more solid givens?

StupendousMan
2011-Sep-28, 11:58 AM
All important items, no doubt. What about extinctions, since SDSS is terrestrial, or are the results already atmospherically compensated?


The SDSS results have been corrected to remove (most of) the effects of the Earth's atmosphere.



In earlier post, I did offer a correction (Sandage) to the Tollman inverse 4th power term, assuming I'm even on the same page of what is being addressed. Was I wrong? Won't the equation be important to resolving some of the questions?


Your earlier posting mentioned "Lobin and Sandage 2001." I believe you were referring to a series of a 4 papers by Lubin and Sandage, the last of which is

http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001AJ....122.1084L&db_key=AST&link_type=ABSTRACT&high=4cead52d9b01416

In that series of papers, the authors look at the empirical relationship between surface brightness in one particular passband and redshift. They fit the results to models which look like (surface brightness) = K (z)^n, and find values of the exponent "n" which are less than 4 for both R and I passbands. This is not inconsistent with the Tolman hypothesis, since the properties of galaxies observed in a fixed passband will change with redshift. Tolman's formula is an ideal one, which would apply if one could move a single, constant source throughout the universe at will, and measure its bolometric energy output (measure all energy emitted at all wavelengths). As Lubin and Sandage conclude in their abstract,


We conclude that the Tolman surface brightness test is consistent with the reality of the expansion to within the combined errors of the observed <SB> depression and the theoretical correction for luminosity evolution.






Wouldn't heavier intergalactic neutral hydrogen be more prevalent in earlier periods disrupting the Lyman break observations?


No.



Here's a wild one -- Has anyone produced a SED of a region of "empty" space to see the "color of the sky"? A great deal of scattering takes place, which might help reveal more of the light sources. [Is my heliochromology showing? :)]


Yes, there have been several papers describing the spectrum of "blank" regions of the sky. I recall seeing one such paper, which described scattered light within the Milky Way ... ah, here it is:

http://arxiv.org/abs/1109.4175




What of metalicity of early galaxies as it relates to anticipated spectrums?


There will be small changes to the spectra of galaxies at high redshift due to the lower metallicity, but those are minor compared to the large changes in the spectra due to the younger stellar populations.

Nereid
2011-Sep-28, 12:50 PM
You can picture an object-by-object approach, if you start by having images of a galaxy at a number of wavelengths. In a simple case, where you picked one for which you have a UV image that would redshift to a desired optical band at a redshift of interest, you would calculate the flux your galaxy would have at the new redshift along with the change in its angular size ("aberration" and all), then make a version of that image with appropriate noise. As an example, take a GALEX near-UV image of some galaxy at z~0.05, consider what it would look like at z-0.25 (HDF F30W) or z=0.87 (HDF F450W) at the angular resolution and noise per pixel of the HST HDF (or other) observations. How much of the galaxy if any) passes a desired S/N threshold? This lets you sample behavior at narrow redshift ranges but needing no assumptions about the spectral behavior, since that's all implicit in the images. The point of doing this for local galaxies which have lots of longer-wavelength data is that you then sort of understand where they fall in optical luminosity, Hubble type, concentration index, and so on, so you can ask how the mix of these entering a catalog changes with redshift.
:cool:

Would it be accurate to say that Disney&Lang might claim that the Tolman dimming was not (adequately) accounted for in this approach?


There were cottage industries doing this when good UV images first became available for lots of galaxies (a major program on the Astro-2 mission, for example, even before HST could do it well). Many show up doing an ADS abstract-word search on "morphogical K-correction".
I see many enjoyable hours ahead of me, spent reading some of these papers! Thanks. :)


As far as I can tell, the pieces of cosmology that enter are the angular size-redshift relation, and whether time dilation follows (1+z). The latter comes in by increasing the mean time between photon arrivals. Heuristically, one can thin of the Tolman dimming as having one factor of (1+z) for photon energy, one for photon arrival time, and two for increase in angular size of distant objects compared to the Euclidean case (what Disney and co. call aberration). Additional terms may enter observationally but are purely detector-based; for example, the width of a filter band in the emitted frame decreases as 1/(1+z), but you could in principle avoid that by using a new filter which matches some standard one when used at a particular redshift. (If you know the spectrum, this can be corrected in a purely numerical way).
There's one other that I had in mind, if we were to use TvD11: reversing their calculation of (LRG-frame) radial distances ... the x-axis in their Figure 6, for example, is in kpc (not arcsecs).

Nereid
2011-Sep-28, 01:03 PM
StupendousMan has already commented on much of your post George; some additional comments ...
I feel like a kid going to the zoo for the first time. The nuances of distant galaxy exploring is new to me.
Well I think this is very interesting and exciting stuff, but was expecting to be going solo on the exploration. So I was glad StupendousMan said "An open discussion on this thread might be interesting and educational for many readers" :)


In earlier post, I did offer a correction (Sandage) to the Tollman inverse 4th power term, assuming I'm even on the same page of what is being addressed. Was I wrong? Won't the equation be important to resolving some of the questions?
Did StupendousMan identify the paper you had in mind?


Other questions I have are crude:
I don't know about crude; as long as you're learning (and enjoying yourself), all questions are good, aren't they?


Wouldn't heavier intergalactic neutral hydrogen be more prevalent in earlier periods disrupting the Lyman break observations?
Evolutionary changes - of any kind - are certainly interesting.

However, Disney&Lang's main point is that the galaxies we see, at redshifts of ~0.5+, cannot possibly be 'like' the ones which are so prominent locally (M31, M101, M87, M82, ...), in a universe with "aberration" and Tolman dimming. What I'm interested in doing is exploring their hypothesis (or hypotheses, I suspect there's actually more than one), and testing it (them). Starting with 'assume aberration and Tolman dimming, but no galaxy evolution'.


[I'm reluctant to ask questions that might slow the pace.]
Please don't be reluctant!


Yes, what are the more solid givens?
How well did you understand ngc3314's most recent post?

Nereid
2011-Sep-28, 01:12 PM
What of metalicity of early galaxies as it relates to anticipated spectrums? There will be small changes to the spectra of galaxies at high redshift due to the lower metallicity, but those are minor compared to the large changes in the spectra due to the younger stellar populations.
This ties back to several things ngc3314 said, starting from the OP:

"gains from the very blue colors of young stellar populations, detection of galaxies from their UV-bright compact regions at substantial redshift, "

"This is what would make Milky Way analogs "drop out" at higher redshifts, although the details depend on how we observe them - higher resolution lets us see the bright cores and star-forming regions, while redshifting ultraviolet into the visible band would exacerbate light loss due to dust while presenting us brighter and more compact star-forming regions when they are not heavily dust-reddened."

"the maximum surface brightness of starburst regions in galaxies is pretty constant with z when taking Tolman dimming into account, although the linear scale and thus luminosity changes a lot."

Nereid
2011-Sep-29, 04:02 PM
This thread's been a bit quiet; loss of interest?

It might be helpful, for lurkers and others, to get a bit of a feel for the SDSS filters/bandpasses and redshift.

For simplicity, consider this: what redshift would an object have for light emitted in one bandpass to be seen in the next (longer wavelength) one? Of course, to answer this in detail is quite complicated, not least because the central wavelengths of each bandpass are not related to each other in a simple way, nor are the detector response plus filter transparency a square function! But at what redshift is light emitted at 354.3 nm (centre of the u-band) observed at 477 nm (centre of the g-band)? 0.346. Here are redshifts for g->r, r->i, and i->z, respectively: 0.306, 0.224, 0.198.

Crudely, then, for the 'average' LRG in TvD11 - with a redshift of 0.34 - the g-band light profile (in Figure 6) is the u-band one, in the rest frame (i.e. as if it were observed at zero redshift), and the r the g*.

The centre wavelengths of the two GALEX filters are 227.5 nm (NUV) and 155.0 nm (FUV); the TvD11 average LRG u-band profile is not really the same as a zero redshift NUV one (264 nm becomes 354 nm, when redshifted 0.34), but it's close.

Now for a research question: given the light profiles in Figure 6, how to convert them into 'zero-z' ones? For simplicity, leave the passbands alone (i.e. derive curves for an SDSS u-band (g-band, r-band, ...) blueshifted by 0.34); just 'reverse out' the (1+z) "aberration"** and the (1+z)^-4 Tolman dimming.


* except for "aberration" and Tolman dimming, of course!
** I don't like using this term; does anyone have a better, equally pithy, alternative?

ngc3314
2011-Sep-29, 05:11 PM
Quick (airport) post on something George brought up. At the highest redshifts we can reach (z>6), the amount of neutral H in the intergalactic medium does matter. As we finally see the hydrogen Gunn-Peterson effect completely block light shortward of redshifted Lyman alpha, the effective location of the Lyman break shifts from 912 to 1216 A (plus a bit for line broadening) in the emitted frame, which can now be modeled realistically since we have QSOs and GRBs to show us how fast this happens as a function of redshift..

George
2011-Sep-29, 07:38 PM
The SDSS results have been corrected to remove (most of) the effects of the Earth's atmosphere. Thanks, and thank goodness. :)


Your earlier posting mentioned "Lobin and Sandage 2001." I believe you were referring to a series of a 4 papers by Lubin and Sandage, the last of which is

http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001AJ....122.1084L&db_key=AST&link_type=ABSTRACT&high=4cead52d9b01416

In that series of papers, the authors look at the empirical relationship between surface brightness in one particular passband and redshift. They fit the results to models which look like (surface brightness) = K (z)^n, and find values of the exponent "n" which are less than 4 for both R and I passbands. This is not inconsistent with the Tolman hypothesis, since the properties of galaxies observed in a fixed passband will change with redshift. Tolman's formula is an ideal one, which would apply if one could move a single, constant source throughout the universe at will, and measure its bolometric energy output (measure all energy emitted at all wavelengths). As Lubin and Sandage conclude in their abstract,... I assumed that at the time of the formula involving z that the age of the universe, Hubble constant, and other factors have been better refined such that the 4th power got tweaked by Sandage et. al., but apparently only for the R & I band. [Free time has become in short supply, but this weekend might grant me time to read-up on this and the tweaking, assuming tweaking is the right word.]


Yes, there have been several papers describing the spectrum of "blank" regions of the sky. Wow, that's great.


I recall seeing one such paper, which described scattered light within the Milky Way ... ah, here it is:

http://arxiv.org/abs/1109.4175
Another something I must look at.

The thought is that scattering might prove to be a helpful indirect tool of the deeper regions, if we can tickle out the foregound. [I know an astronomer that added the known atmospheric spectral extinctions to the sp. irr. of the Sun and matched it to a star -- I found which one it was, too: Procyon (F5) -- then observed its color as seen from Kitt Peak. By observing Procyon terrestrially (after extinctions) he was, in effect, seeing the Sun's true color. Such spectral manipulations I know are common, but I thought I could combine the spectral scattering idea with adding a grin to this same astronomer who is reading this. :)

Perhaps scattering information might be another source of evidence to help discern all those ancestral species.

BigDon
2011-Sep-30, 01:30 PM
Still reading. Trouble is every read through I come away with something different.

But I will say...

Have you tried running this issue passed people skilled in electronic warfare? Especially "active" electronic warfare? I knew several back in the day who would have been fascinated with this issue. Hiding entire galaxies and dynasties of galaxies through signal manipulation! Which is "all" you are dealing with. Instead of ionispheric bounces, reflective chaff and intentionally designed false return signals you have the gravity of galactic clusters, the expansion of the universe and local noise bending and altering the light to the point we don't receive it.

It's the same issue just 10^23 orders of magnitude larger.

I think a meeting of the minds between people working on this issue and the folks at NAESU (NAY-su) would benefit both of you.

Nereid
2011-Oct-03, 10:02 PM
Thread's gone quiet again ...

Continuing with TvD11; Figure 2 has a convenient scale bar, in which 50" is equated with 215 kpc (presumably at z = 0.34). That'll be helpful, at least at the level of a first analysis.

Looking at Figure 6, at z = 0.34, the surface brightness (mag/arcsec^2) at ~2.3" from the centre of our 'typical' LRG, is 26, 24, ~22-23, for the u-, g-, r-, i-, and z-bands, respectively.

Reversing out the "aberration", this means that surface brightness (in the same units) will be these same values at 1.7", in the blueshifted u-, ... z-bands (1.7 = 2.3/(1.34)).

Yes? No? Maybe? Don't know??

Next: the (1+z)^-4 Tolman dimming, applied to the same data ...

ngc3314
2011-Oct-04, 03:21 AM
Thread's gone quiet again ...

Continuing with TvD11; Figure 2 has a convenient scale bar, in which 50" is equated with 215 kpc (presumably at z = 0.34). That'll be helpful, at least at the level of a first analysis.

Looking at Figure 6, at z = 0.34, the surface brightness (mag/arcsec^2) at ~2.3" from the centre of our 'typical' LRG, is 26, 24, ~22-23, for the u-, g-, r-, i-, and z-bands, respectively.

Reversing out the "aberration", this means that surface brightness (in the same units) will be these same values at 1.7", in the blueshifted u-, ... z-bands (1.7 = 2.3/(1.34)).

Yes? No? Maybe? Don't know??

Next: the (1+z)^-4 Tolman dimming, applied to the same data ...

The Tolman signal includes the effects of "aberration", so it's evaluated at the same radii for like objects. (BTW, I also wish there were a better term than "aberration" for "departures from Euclidean angular size-distance relation").

The way I would view this might be that the surface brightnesses in shifted bands matching emitted wavelength and projected radius would all be brighter by [2.5 log (1.34^4)] = 1.27 magnitudes at z=0. Using magnitudes is tricky, since various magnitude systems have essentially arbitrary zero points tied to particular spectral responses (so working in F-lambda, or using a well-established set f K-corrections to account for the spectral shape, is needed).

Nereid
2011-Oct-05, 01:57 PM
For the benefit of lurkers, I think a quick refresher on some key terms might be helpful.

The various 'magnitudes' used in SDSS (DR8) are described here: Measures Of Flux And Magnitude (http://www.sdss3.org/dr8/algorithms/magnitudes.php).

Now several terms are sprinkled liberally throughout that webpage (and in many others on observational astronomy), e.g. 'flux', 'luminous flux', 'intensity', 'surface brightness', 'luminosity'.

There's also reference to 'profiles' (some of the magnitudes the photometric SDSS pipeline produces involve 'fits', using one profile or another); here is a quick primer on the general topic of the distribution of observed 'brightness', within the galaxy, of various (classes of) galaxies, and various widely used models thereof: Structural Components of Galaxies (http://www.astr.ua.edu/keel/galaxies/components.html) (here is another, focussing on ellipticals: Properties of Elliptical Galaxies (http://www.ifa.hawaii.edu/~barnes/ast626_97/peg.html)).

So, how are all these things related? Here is one quick backgrounder: Intensity, Flux Density and Luminosity (http://www.starlink.rl.ac.uk/star/docs/sc6.htx/node8.html)*, and the next page, Magnitudes (http://www.starlink.rl.ac.uk/star/docs/sc6.htx/node9.html).

So, in terms of the physics, (observational) astronomers are measuring (and discussing) energy of something, per something (or some things): of detected electromagnetic radiation, per unit time (so it's more power than energy), per unit wavelength (or per unit frequency), per unit solid angle, ... and they often do so in terms of a unit ('magnitude') that's sometimes difficult to tie back to energy cleanly. IOW, it's not always straight-forward to convert an observational astronomer's 'intensity' to Janskys (Jy, 1 Jy = 10^-26 W m^-2 Hz^-1).

* One fly in the ointment: "The geometry of the situation results in the interesting fact that the observed surface brightness is independent of the distance of the observer from the extended source" (at the bottom of the first of those pages); one of the key parts of the Disney&Lang paper, and of any research into is, concerns the fact that, in a GR-dominated universe, there are (in general) "departures from Euclidean angular size-distance relation" (to quote NGC3314).

Nereid
2011-Oct-06, 09:19 PM
Here (http://cas.sdss.org/astro/en/tools/explore/obj.asp?sid=281701078595010560) is a DR7 spectrum of a galaxy (from SDSS, obviously).

Note the label on the y-axis: "Fλ [10-17 erg cm-2 s-1 ┼-1 ]". Clearly this is flux, with dimensions of ergs per second (so that's power, not energy), per square cm, per Angstrom (which is 0.1 nm) ... and a constant.

Would any reader (other than StupendousMan, NGC3314, or parejkoj) like to have a go at expressing "10" of these flux-per-Angstrom as a (Pogson) magnitude?

ETA: Here (http://skyserver.sdss3.org/dr8/en/tools/explore/obj.asp?id=1237658300608676014) is the DR8 spectrum of the same galaxy.

George
2011-Oct-07, 02:09 AM
Here (http://cas.sdss.org/astro/en/tools/explore/obj.asp?sid=281701078595010560) is a DR7 spectrum of a galaxy (from SDSS, obviously). Having read your prior post, I'm now bedazzled on what those magnitudes really represent for each band.


Note the label on the y-axis: "Fλ [10-17 erg cm-2 s-1 ┼-1 ]". Clearly this is flux, with dimensions of ergs per second (so that's power, not energy), per square cm, per Angstrom (which is 0.1 nm) ... and a constant. Right, I get this. Integration would give you the actual power amount we receive.

The example spectrum, if it were a star, would be somewhat close to a Planck temp. of roughly 4,300K -- compensating for redshift. I have no idea, of course, what value this might have in reprsenting galaxies, but it's something....maybe.


Would any reader (other than StupendousMan, NGC3314, or parejkoj) like to have a go at expressing "10" of these flux-per-Angstrom as a (Pogson) magnitude? Right! We don't need no stinkin' astronomer. What good's a vulcanist to a dive-bombin' moth? :)

George
2011-Oct-07, 03:10 AM
Tolman Diming mentioned from the start is intriguing.

I went to New Wright's calculator web site and plotted this. [I don't know if he included abberation in his equations.]

15449

Nereid
2011-Oct-07, 06:21 AM
Here (http://cas.sdss.org/astro/en/tools/explore/obj.asp?sid=281701078595010560) is a DR7 spectrum of a galaxy (from SDSS, obviously).

Note the label on the y-axis: "Fλ [10-17 erg cm-2 s-1 ┼-1 ]". Clearly this is flux, with dimensions of ergs per second (so that's power, not energy), per square cm, per Angstrom (which is 0.1 nm) ... and a constant.

Would any reader (other than StupendousMan, NGC3314, or parejkoj) like to have a go at expressing "10" of these flux-per-Angstrom as a (Pogson) magnitude?

ETA: Here (http://skyserver.sdss3.org/dr8/en/tools/explore/obj.asp?id=1237658300608676014) is the DR8 spectrum of the same galaxy.

By chance, I was browsing astro-ph, and came across The Cosmic Origins Spectrograph (http://arxiv.org/abs/1110.0462).

The abstract includes this: "For faint targets, with flux F_lambda ~ 1.0E10-14 ergs/s/cm2/Angstrom, COS can achieve comparable signal to noise (when compared to STIS echelle modes) in 1-2% of the observing time." And I thought, Huh?!? the galaxy whose SDSS spectrum I linked to surely isn't 'faint', yet from ~6000 to 9000 ┼, F_lambda is ~1.3E10-16 ergs/s/cm2/Angstrom!

Reading the paper, I think I can see why SDSS galaxy spectra can be easily two orders of magnitude brighter than COS* ones (in F_lambda):

By eliminating windows in the detector systems, the throughput is enhanced and the noise level is greatly reduced, resulting in large gains in signal to noise, particularly for the faintest targets with Fλ < 1.0 Î 10−14 erg cm−2 s−1 ┼−1 at 1200 ┼

BTW, the galaxy is SDSS J104155.65+074513.9 (http://cas.sdss.org/astro/en/tools/explore/obj.asp?sid=281701078595010560), and CAS gives its photometric pipeline outputs as:
fiberMag_r 18.62 mag
petroMag_r 17.22 mag
devMag_r 17.07 mag
expMag_r 17.41 mag
psfMag_r 18.60 mag
modelMag_r 17.07 mag
petroRad_r 5.810 arcsec

* interestingly, the COS 'aperture' is 2.5", close to that of the SDSS spectrograph (which is 3")

George
2011-Oct-07, 12:54 PM
The abstract includes this: "For faint targets, with flux F_lambda ~ 1.0E10-14 ergs/s/cm2/Angstrom, COS can achieve comparable signal to noise (when compared to STIS echelle modes) in 1-2% of the observing time." And I thought, Huh?!? the galaxy whose SDSS spectrum I linked to surely isn't 'faint', yet from ~6000 to 9000 ┼, F_lambda is ~1.3E10-16 ergs/s/cm2/Angstrom! Approximating the integration, that's a little less than about 3,000 photons per sec. [Sunlight at 1 AU is about thousand trillion times greater in flux (~ 1e18 photons per sec).]

ngc3314
2011-Oct-07, 02:57 PM
COS works in the UV (where detectors still don't approach the 90% of a good CCD in the optical), and is designed specifically to use higher dispersion than in the SDSS - they mention 20 km/s in Doppler width which maps to roughly 0.05 A pixels, assuming Nyquist 2-pixel sampling, rather than the ~2 A/pixel of SDSS spectra.

Jerry
2011-Oct-07, 05:15 PM
Jerry, George, and BigDon (and any other interested reader lurker): do you think you could sketch the steps you think should be taken to work out an answer?
You a proposing a freshman astronomy approach to a problem that is much more complex than calculating the sky drop-out in different bandwidths.

Look at the galaxies that we can see in the ultra deep Hubble fields. they are warped, torn and distorted. The lensing elements are both gravimetric and (likely) distorted by many pockets of intervening dust and gas. The Lyman forest tells us that the light is reaching us in tatters.

Freshman calculations aside, the best we could hope for, is that the most distant events are directly mirrored by fairly local events and we can then make statistical assumptions based upon this premise. But current theory also dictates an evolution element; which is 'confirmed' by the fact that all of our freshman calculations indicate galaxies were brighter in the past than they are today. We can accept this at face value; but as our depth of knowledge about the spectral features of these most distant observations become clearer; if their root structure, metallicity and other evidences of evolution are found wanting; we should wonder how this new evidence fits with our best prior explanations.

Years ago, someone on this board discribed a scientist as someone who immediately explains the reason behind something that they told you was impossible in last year's lecture. This year, we are giving a Nobel prize this to three scientists who 'found' what Einstien called his biggest blunder. Something is missing in this equation, throwing in another ad hoc parameter is not the best solution.

What do I propose? I have already stated several times: Start with a shotgun of initial of possible theories and run the light through all the different wringers: Which one provides the best match with all of the modern evidence? Don't forget to remove your deeply ingrained preconceptions - which have been proven impossible - from your analysis.

StupendousMan
2011-Oct-07, 07:46 PM
What do I propose? I have already stated several times: Start with a shotgun of initial of possible theories and run the light through all the different wringers: Which one provides the best match with all of the modern evidence? Don't forget to remove your deeply ingrained preconceptions - which have been proven impossible - from your analysis.

So, go ahead and do what you propose. No one is stopping you.

ngc3314
2011-Oct-07, 08:05 PM
What do I propose? I have already stated several times: Start with a shotgun of initial of possible theories and run the light through all the different wringers: Which one provides the best match with all of the modern evidence?

That is effectively an infinite universe of starting theoretical possibilities. Your low opinion of the intellect and intellectual honesty of my entire profession is noted, but surely you could propose a realistically attainable alternative?

Nereid
2011-Oct-07, 09:33 PM
Approximating the integration, that's a little less than about 3,000 photons per sec. [Sunlight at 1 AU is about thousand trillion times greater in flux (~ 1e18 photons per sec).]
Can you expand a little please?

I'm not 100% sure what the "that" refers to (the COS number, or the SDSS example?); and in any case, I can't see how you arrived at the 3,000 photons/sec conclusion (the detectors in the SDSS spectrograph, and COS, certainly aren't 100%, especially after the losses before photons hit them, but 'faint = 3,000 photons/sec' seems wrong).

One heuristic I've heard: 0 mag is 10,000 photons/sec at the top of the atmosphere, at 5500 ┼ (per square cm, per ┼). I've no idea how accurate it is (can we check it?), nor where it comes from, nor how widespread it is (among observational astronomers, in the 'optical').

ngc3314
2011-Oct-07, 09:53 PM
C
One heuristic I've heard: 0 mag is 10,000 photons/sec at the top of the atmosphere, at 5500 ┼ (per square cm, per ┼). I've no idea how accurate it is (can we check it?), nor where it comes from, nor how widespread it is (among observational astronomers, in the 'optical').

The rule I heard from Joe Wampler in grad school was V=0 corresponds to 1000 photons/(cm2 second Angstrom) in the middle of the V band. I once did a more careful calculation and got something like 1090, but adding significant figures starts to need so many details (spectral shape, filter passband) that it ceases to have very general use.

Nereid
2011-Oct-08, 07:35 AM
Jerry, George, and BigDon (and any other interested reader lurker): do you think you could sketch the steps you think should be taken to work out an answer?You a proposing a freshman astronomy approach to a problem that is much more complex than calculating the sky drop-out in different bandwidths.
You say this as if it's not worth doing, even as the first part to a more thorough examination of the question.

You also seem to have not really understood what I was outlining; may I ask how you came to conclude I was proposing to (merely) calculate "the sky drop-out in different bandwidths"


Look at the galaxies that we can see in the ultra deep Hubble fields. they are warped, torn and distorted.
Some are, some aren't.

How is this relevant to what I suggested, may I ask?


The lensing elements are both gravimetric and (likely) distorted by many pockets of intervening dust and gas.
Are they? How did you arrive at that conclusion, may I ask?


The Lyman forest tells us that the light is reaching us in tatters.
Colourful analogy; how is it helpful, may I ask?


Freshman calculations aside, the best we could hope for, is that the most distant events are directly mirrored by fairly local events and we can then make statistical assumptions based upon this premise.
It is? If you haven't done the work - of the kind I outlined - how would you know?


But current theory also dictates an evolution element; which is 'confirmed' by the fact that all of our freshman calculations indicate galaxies were brighter in the past than they are today.
I think you may not have really understood what you've been reading.

In any case, you seem to be saying that it's simply not even worth doing the kind of research I sketched; is that what you're saying?


We can accept this at face value;
Um, how, may I ask, did you so badly miss the central point of my posts?


but as our depth of knowledge about the spectral features of these most distant observations become clearer; if their root structure, metallicity and other evidences of evolution are found wanting; we should wonder how this new evidence fits with our best prior explanations.
It's like you're reading an 18th century novel, and I'm reading a 20th century thesis dissertation! :razz:

What has this got to do with testing Disney&Lang's ideas (per their paper)?

Do you think it a complete waste of time to test them?


Years ago, someone on this board discribed a scientist as someone who immediately explains the reason behind something that they told you was impossible in last year's lecture. This year, we are giving a Nobel prize this to three scientists who 'found' what Einstien called his biggest blunder. Something is missing in this equation, throwing in another ad hoc parameter is not the best solution.
And you've been saying things like this for years too.

When, may I ask, do you think you'll get down from your soapbox, roll up your sleeves, and start doing some real work?


What do I propose? I have already stated several times: Start with a shotgun of initial of possible theories and run the light through all the different wringers: Which one provides the best match with all of the modern evidence? Don't forget to remove your deeply ingrained preconceptions - which have been proven impossible - from your analysis.
OK, got it.

You clearly think the research I've suggested - in my various posts in this thread - is a complete waste of your time. Thanks for letting us know. Please don't waste any more of your time even reading the posts in this thread. Instead, why not concentrate on getting your proposal (now many years' old) into a form that you could - at some future time - start to think about how you could, perhaps, in another two decades or so, begin to test it?

Nereid
2011-Oct-08, 07:53 AM
By chance, I was browsing astro-ph, and came across The Cosmic Origins Spectrograph (http://arxiv.org/abs/1110.0462).

The abstract includes this: "For faint targets, with flux F_lambda ~ 1.0E10-14 ergs/s/cm2/Angstrom, COS can achieve comparable signal to noise (when compared to STIS echelle modes) in 1-2% of the observing time." And I thought, Huh?!? the galaxy whose SDSS spectrum I linked to surely isn't 'faint', yet from ~6000 to 9000 ┼, F_lambda is ~1.3E10-16 ergs/s/cm2/Angstrom!

Reading the paper, I think I can see why SDSS galaxy spectra can be easily two orders of magnitude brighter than COS* ones (in F_lambda):


BTW, the galaxy is SDSS J104155.65+074513.9 (http://cas.sdss.org/astro/en/tools/explore/obj.asp?sid=281701078595010560), and CAS gives its photometric pipeline outputs as:
fiberMag_r 18.62 mag
petroMag_r 17.22 mag
devMag_r 17.07 mag
expMag_r 17.41 mag
psfMag_r 18.60 mag
modelMag_r 17.07 mag
petroRad_r 5.810 arcsec

* interestingly, the COS 'aperture' is 2.5", close to that of the SDSS spectrograph (which is 3")
From here (http://www.noao.edu/kpno/mosaic/filters/filters.html), the SDSS r-band filter has a FWHM of ~147 nm, and a central wavelength of ~629 nm.

SDSS J104155.65+074513.9's spectrum is more or less flat in the r-band; suppose F_lambda is 1.3E10-16 ergs/s/cm2/Angstrom throughout*; that gives it an r-band flux (or flux density!) of ~1.9E10-13 ergs/s/cm2.

From the SDSS DR8 page (http://www.sdss3.org/dr8/algorithms/magnitudes.php) I linked to earlier:

A "maggy" is the flux f of the source relative to the standard source f0 (which defines the zeropoint of the magnitude scale). Therefore, a "nanomaggy" is 10-9 times a maggy. To relate these quantities to standard magnitudes, an object with flux f given in nMgy has a Pogson magnitude:

m = [22.5 mag] − 2.5 log 10 f .

[...]

The standard source for each SDSS band is close to but not exactly the AB source (3631 Jy), meaning that a nanomaggy is approximately 3.631Î 10-6 Jy.
Almost there; what's an r-band flux (or flux density!) of ~1.9E10-13 ergs/s/cm2 expressed as an r-band (Pogson) magnitude (approximately)?

* it isn't, of course, but let's assume that cows are spherical

StupendousMan
2011-Oct-08, 08:08 PM
The numbers posed earlier in this thread are off by quite a bit. It's not so hard to do the calculations -- I did it with students in class earlier this week.

A star of magnitude V = 0 has a flux density of roughly 3.6 x 10^(-9) ergs per sq. cm. per second per Angstrom. You can find this value in Allen's "Astrophysical Quantities", in other good textbooks, in many many websites, such as

http://www.sr.bham.ac.uk/~somak/constants.html

So, what does that mean? It gives the number of ergs of energy which will be collected each second if one points a telescope with 1 sq. cm. of collecting area at a star of magnitude zero, with a filter 1 Angstrom wide. Let's convert to the number of photons per second for that same telescope. The V-band is roughly -- very roughly -- 1000 Angstroms wide. The central wavelength of light in the V filter is roughly 5500 Angstroms. Now, physics tells us that the energy associated with a photon of wavelength lambda is h*c/lambda, where h = Planck's constant, c = speed of light, and lambda = wavelength in meters.

h * c / (lambda) = 6.626x10^(-34) kg m^2/s * 3 x 10^8 m/s / (5500 x 10^(-10) m) = 3.6 x 10^(-19) Joules

= 3.6 x 10^(-12) ergs

Okay. So, if we start with the flux density from a star of magnitude V = 0, we can compute the number of photons which will be collected by a telescope of collecting area 1 sq.cm. each second:

3.6 x 10^(-9) erg / (sq.cm.*s*Angstrom) * 1000 Angstrom / ( 3.6 x 10^(-12) erg/photon ) = 1 million photons

So, the rule to remember is that a star of mag V = 0 will yield about 1 million photons per second per square cm of collecting area. Well, above the atmosphere, and assuming perfect optics and detector, yadda yadda yadda.

Look at the galaxy mentioned in the post above: the "faint" galaxy is said to have a spectrum with a flux density of about 10^(-16) ergs per sq.cm. per second per Angstrom. That's roughly 10^7 times smaller than the flux of a V = 0 star. That means that the magnitude of that galaxy must be roughly 2.5 * log10 (10^7) = 17.5. Yup, that's a pretty faint galaxy.

This stuff is covered in advanced undergraduate observational astronomy courses -- not that there are many of those -- and graduate courses. Actually, a lot of it is learned on the street from colleagues, since there really aren't that many people who need to know in their regular jobs. You can find "exposure calculators" at the websites of many observatories which will do that calculations for you, taking a magnitude and figuring out how many photons per second will reach the detector.

Jerry
2011-Oct-09, 04:38 AM
That is effectively an infinite universe of starting theoretical possibilities. Your low opinion of the intellect and intellectual honesty of my entire profession is noted, but surely you could propose a realistically attainable alternative?

This thread contains a discussion that is WAY TO GOOD to jumble it up with any more cross talk. Sorry for the jabber I will be brief:

The short answer is no. I have worked as exhaustively as I can to find alternatives that fits all of the known facts, and so have many others. But it is important to keep prodding, because many parameters have already been added, and they are not all right. Granting a Nobel prize for throwing in a new constant should not put the stamp of scientific approval on the latest tweak of the nob. Antagonistic science is good science insofar as it incourages an investment in new ideas, new designs, new approaches and most especially, new funding.

I think the lunar gravitational mapping probes (Grail) are an excellent example of how asking the right questions has lead to a new and clever test.

We need more of the same.

George
2011-Oct-09, 06:37 AM
Can you expand a little please?
Here is what I did, though I see I goofed with the math, but the procedure is possibly correct if you only want a rough value. If I had the data set, I could do a more respectable job, if for some reason such a result would be useful.

Since E=hc/lambda , then the “blue” photons will have twice the energy of the far “red” ones, which are twice the wavelength. This will squash the blue end of the spectrum if we convert the curve to a photon flux curve. The peak shifts to the red. [The Sun’s shift is from 495nm (using a Planck peak @ 5850K -- the real peak is closer to 450nm, surprisingly) for a wattage curve to 695nm to a photon flux curve peak.]

As I understand it, and without an appropriate book or teacher, the integration (ie area) of the curve represents the flux density because the power stated for the y-axis is per unit wavelength. [Be sure to extrapolate into the IR band to get the bulk of the complete spectrum.] So what we want out of this spectrum is the power (ergs per second) value that is a result of the integration. The Sun, for instance, has a peak sp. irr. value of over 2,100 watts m-2 @ 1 AU, but we know the actual wattage (ie Solar Constant) is only ~ 1,361 watts m-2 @ 1 AU. So, in the Sun’s case, the value is 64% of the peak.

Arbitrarily using this same percentage would give us about 9.5 E -17 ergs s-1 cm-2 for the example galaxy spectrum.

Now choose a wavelength that best represents the place the flux distribution is balanced. Say it is about 1500nm, which has a photon energy of 1.32E-12 ergs. So dividing this energy into the power above (per cm-2) yields a photon flux rate of 7.20E-5 per cm -2. This means you would need a 1.3 meter aperture scope to get one photon per second.

Obviously, this is a very rough value given the lack of effort to get hard values, but it should be a reasonable approach to the problem.

It is much too late to study the other related posts by the more credible posters, but one concern I have regarding Vega is that it is a “blue” star (9600K) so there will be fewer photons coming from it compared to a red star, for instance, having the same apparent magnitude -- perhaps by a factor of 2 to 5, which isn't much of a variance I suppose.

I hope the others will review this approach since it is just something I did on a wing. [It was a wing that stayed on the ground for a long time, admittedly. :)]

George
2011-Oct-09, 06:58 PM
The rule I heard from Joe Wampler in grad school was V=0 corresponds to 1000 photons/(cm2 second Angstrom) in the middle of the V band. I once did a more careful calculation and got something like 1090, but adding significant figures starts to need so many details (spectral shape, filter passband) that it ceases to have very general use. That makes sense given the value StupendousMan gives us in his post, namely 3.6 x 10^(-9) ergs per sq. cm. per second per Angstrom for V=0, (within the V band, not the entire spectrum).

As he did, 550nm does give us 3.6 x 10^(-12) ergs per photon, so the simple division does give 1000 photons per second per cm. I do think, however, that the SED of the object will vary the flux since fewer blue photons are needed to carry the same amount of energy carried by red ones.


Okay. So, if we start with the flux density from a star of magnitude V = 0, we can compute the number of photons which will be collected by a telescope of collecting area 1 sq.cm. each second:

3.6 x 10^(-9) erg / (sq.cm.*s*Angstrom) * 1000 Angstrom / ( 3.6 x 10^(-12) erg/photon ) = 1 million photons. [my bold] Multiplying by the bandwidth will not work for SEDs because integration is required to determine the actual flux density. At least I am fairly sure of this. In other words, the y-axis values are in some sort of derivative form when they use the per wavelength term. [There is likely a more appropriate way to say this and I'd like to know what it is, assuming I'm right, of course.]

ngc3314
2011-Oct-09, 07:07 PM
[my bold] Multiplying by the bandwidth will not work for SEDs because integration is required to determine the actual flux density. At least I am fairly sure of this.

and to add one more complication (that really does seem to be talked on the street but seldom written): when the detector counts photons, you have to do the integration after converting the spectrum into photon units, because, for example, a CCD counts one photon (i.e. one electron at readout) no matter what the photon energy is, so that a blue photon has more leverage in energy space. This matters in a differential sense when the spectral shapes of two objects being compared are very different; the mean energy of V photons from a red object is distinctly smaller than for a blue object. This matters less for narrow bands and more for broad bands; for years, X-ray astronomy largely consisted of modeling the range of contributions of a few models allowed by an instrumental response that was an order of magnitude wide in energy. (No, in comparison to that, the magnitude scale in optical astronomy introduces no complications...)

George
2011-Oct-09, 07:42 PM
and to add one more complication (that really does seem to be talked on the street but seldom written): when the detector counts photons, you have to do the integration after converting the spectrum into photon units, because, for example, a CCD counts one photon (i.e. one electron at readout) no matter what the photon energy is, so that a blue photon has more leverage in energy space. This matters in a differential sense when the spectral shapes of two objects being compared are very different; the mean energy of V photons from a red object is distinctly smaller than for a blue object. So you are saying that the electrons aren't colorful enough? ;) [If blue and red electrons emerged from sensors, life would be easier.]

I think I see your point. If all you have is an electron count for a given sensor reading, say within the V-band, then how would one know what percent were from "blue" photons and what percent were from "red" ones, as well as the other colors? Even if we knew the reactiviness of blue ones vs. red ones with the sensor, we still wouldn't know what the actual spectral flux was. If, however, we already knew the object's surface temperature we could apply a Planck distribution to the spectral sensitivity of the sensor and make the adjustments. Am I close? [It's not quite as bad as, "If we had some ham, we could have ham and cheese, if we had some cheese. :)]

Nereid
2011-Oct-09, 11:08 PM
Obviously, my memory isn't what it used to be. :cry:

At a level much below that we are concerned with in this thread (i.e. ~1%), there are - even for SDSS - unresolved issues (http://www.sdss3.org/dr8/algorithms/fluxcal.php#SDSStoAB). The AB system is what SDSS uses* (Oke & Gunn (1983) (http://adsabs.harvard.edu/abs/1983ApJ...266..713O)); not exactly the same as that in the link in StupendousMan's post?

George: I think there are some potentially very serious challenges to working from a blackbody spectrum, even an 'equivalent temperature' one. Even for the Sun, there's limb darkening, and the fact that, at the limb, a line of sight integrates over different photospheric depths than at the centre, so even if the SED at any depth in the photosphere were a blackbody (which it isn't), the integrated (whole solar disc) SED will not be all that close to a blackbody. When it comes to certain galaxies, this approach can be hopelessly wrong! Consider, for example, SDSS J085604.39+344234.8 (http://cas.sdss.org/astro/en/get/specById.asp?id=341093676589514752): the continuum is vaguely blue, but the narrow emission lines surely totally dominate the SED.

* "by which a magnitude 0 object should have the same counts as a source of Fν = 3631 Jy"

StupendousMan
2011-Oct-10, 12:27 AM
If you want to get the answer right to within a factor of 2 or 3, you can use the quick sort of calculation I demonstrated a few posts back.

If you want to get the answer right to within a few percent, you need to

a) start with a spectrum of the object in question; you can use spectra from Pickles http://cdsarc.u-strasbg.fr/viz-bin/Cat?J/PASP/110/863
b) find a good model for the instrumental bandpass (filter + air + detector ...)
c) break the bandpass into small pieces -- say, 1 Angstrom wide
d) compute the number of photons per sq.cm. per second per Angstrom in each piece
e) add up all the photons from all the pieces

In other words, one must numerically integrate the spectrum across the bandpass. People don't use the blackbody approximation for this sort of calculation, because there's no point to going to all this trouble without using a more accurate spectrum.

StupendousMan
2011-Oct-10, 12:35 AM
At a level much below that we are concerned with in this thread (i.e. ~1%), there are - even for SDSS - unresolved issues (http://www.sdss3.org/dr8/algorithms/fluxcal.php#SDSStoAB). The AB system is what SDSS uses* (Oke & Gunn (1983) (http://adsabs.harvard.edu/abs/1983ApJ...266..713O)); not exactly the same as that in the link in StupendousMan's post?


* "by which a magnitude 0 object should have the same counts as a source of Fν = 3631 Jy"

There are two main types of photometric calibration in the optical: "Vega-based" and "AB". "Vega-based" systems assign a magnitude of zero (or something very close to zero -- you can ask me to explain but it will take a while) to Vega in each passband. So, to a good approximation, Vega has B = 0, V = 0, R = 0, I = 0, etc. This is easy to do in practice, because the calibration is based on a real source that everyone can observe. On the other hand, it's a pain to compute the number of photons per second from a star, or the number of ergs per second, because there's no theory behind the numbers. The number of ergs per second per sq. cm. per second from a V = 0 star isn't the same as the number for an R = 0 star, or for an I = 0 star, etc.

"AB" systems, on the other hand, are more theoretically grounded. The idea is that a star of magnitude zero in any band will produce the same flux density. It is much easier to figure out how to convert from magnitude to flux density in this sort of system, because you only need to know one conversion factor: it will be the same for all passbands. On the other hand, since it is nearly impossible to MEASURE the actual flux density from a star with high precision, these systems have problems assigning magnitudes to stars; there's always the possibility of a significant systematic error. The SDSS magnitude system is one of the AB systems -- Nereid's post provides the zero-point conversion factor for it.

It would be nice if someone would fly a small telescope above the atmosphere and measure the flux density of a few bright stars against some fundamental flux calibration system -- everyone in the entire astronomical community would benefit from this. Unfortunately, funding agencies would rather give money to someone who can "do science" by making measurements of some galaxies or planets or whatever, than give money to calibration efforts. I guess they figure that calibration is unimportant.

George
2011-Oct-10, 04:26 PM
I think there are some potentially very serious challenges to working from a blackbody spectrum, even an 'equivalent temperature' one. The blackbody approach is never going to be very accurate. The closer a given SED is to a Planck distribution, the better the accuracy, but even then it is obvious I took the "shooting from the hip" approach by guessing a weighted midpoint for wavelength, etc.


Even for the Sun, there's limb darkening, and the fact that, at the limb, a line of sight integrates over different photospheric depths than at the centre, so even if the SED at any depth in the photosphere were a blackbody (which it isn't), the integrated (whole solar disc) SED will not be all that close to a blackbody. I assume the SED (Spectral Irradiance for the Sun -- an extended object) for the Sun is an integrated SED, which is why they measure the distribution as an irradiance. [I don't think I ever asked if this is the correct view. Am I right?]


When it comes to certain galaxies, this approach can be hopelessly wrong! Consider, for example, SDSS J085604.39+344234.8 (http://cas.sdss.org/astro/en/get/specById.asp?id=341093676589514752): the continuum is vaguely blue, but the narrow emission lines surely totally dominate the SED. That is a nice example of a SED that if converted to photon flux density would likely be flat as a pancake since the blue end gets cut in half compared to the far red end since the wavelength for blue is only half that of blue. So, perhaps, in this case the incident photon flux will be easier to calculate. But what comes out in electron counts is where the devil seems to be having fun.

I mention the photon flux more as novelty and look to ngc3314 and StupendousMan for the real world stuff.

Nereid
2011-Oct-17, 02:11 PM
What? A whole week has passed?!?

Well, I've been having fun, trying to get my head properly around "flux", "flux density", Fν, Fλ, fν, and fλ, "intensity", "magnitude", "surface brightness", etc, etc, etc. All this as a prelude to being able to start - yep, that's right, merely start - to look at how some actual galaxies behave, when observed by different survey-grade facilities, at different wavelengths. My starting point is (or will be, one day) the Tal and van Dokkum 2011 paper on stacked LRGs I have mentioned several times.

Here is a nice summary, with lots of the key formulae and concepts bundled into a few short (PDF) pages: ASTROPHYSICAL INFORMATION DERIVED FROM THE EM SPECTRUM (http://www.astro.virginia.edu/class/oconnell/astr511/lec2-f03.pdf).

I've also learned a lot about how to extract data from the online SDSS databases, using CasJobs, Yay! :dance:

StupendousMan
2011-Oct-17, 07:31 PM
Way to go, Nereid! Keep up the good work.

It takes a typical grad student 3-6 years to absorb enough of this material to do original research. Sounds like you're ahead of schedule :-)

Nereid
2011-Oct-17, 09:23 PM
Thanks Stupendous Man. :o

I'd like to start with some simplifying assumptions.

The TvD11 stacked LRGs have r, i, and z (μλ vs radius) profiles that are pretty similar; let's assume they are the same [1]. Let's also assume that these galaxies have a smooth continuum all over (i.e. the spectrum taken through a pinhole - circular aperture of 0.01" radius, say - is the same, no matter where on the galaxy it's taken, modulo the integrated flux), no pesky absorption or emission lines to have to worry about. Further assume that the SDSS ugriz magnitudes are AB magnitudes (there is a small difference, but at the ~1-3% level), i.e. their zero points have the same fν. That means - I hope! - that we can move the stacked LRG (treating it as a single galaxy, not an ensemble) closer to us, and further from us, and we can construct an artificial filter/passband/waveband within which colour and surface brightness changes should be easy to handle ... as long as the artificial filter doesn't go longer than the red end of the i-band, or shorter than the blue end of the r-band.

But first, a question: TvD11's Figure 6 y-axis is labelled "μλ [mag/arcsec2]"; however, SDSS ugriz magnitudes are AB magnitudes! Why μλ? Why not μν?

Anyway, in a Euclidean universe, surface brightness is constant, it does not change as the source gets closer or further. If our test LRG is unchanged, we can determine the apparent effective radius, in arcsecs, at any distance, from its (μλ vs radius - in arcsecs) profile (this is true no matter what the geometry of the universe! it follows from the definition of 'effective radius'). In such a universe, then, our trusty LRG becomes marginally detectable, as a galaxy, when its apparent diameter, in arcsecs, is the same as the seeing. And what is its apparent diameter? Twice the distance, in arcsecs, from the nucleus to the point where the surface brightness equals the sky. [2]

So, first, in a Euclidean universe, how big does our LRG appear, at z = 0.01? At what z does it become indistinguishable from a star?

Anyone - other than Stupendous Man, NGC3314, parejkoj, etc - like to try to work that out?

[1] Let's also assume the effective radius is the same, in all three passbands; TvD11 found small, but significant, differences

[2] These two aspects can be tweaked somewhat: a cleverly designed survey may be able to tell the difference between a point source and an extended source somewhat more finely than the average seeing; the outer limit of an object may be traced to somewhat below the sky. These are, however, refinements that we can add later.

Jerry
2011-Oct-18, 02:38 AM
Way to go, Nereid! Keep up the good work.

It takes a typical grad student 3-6 years to absorb enough of this material to do original research. Sounds like you're ahead of schedule :-)
...unless you cheat and read the last chapter first.

StupendousMan
2011-Oct-18, 10:28 AM
Thanks Stupendous Man. :o


But first, a question: TvD11's Figure 6 y-axis is labelled "μλ [mag/arcsec2]"; however, SDSS ugriz magnitudes are AB magnitudes! Why μλ? Why not μν?



It's simply a practical issue. Most passbands in the optical are described in terms of transmission vs. wavelength. So, if one wants to convolve some spectrum with those passbands to compute synthetic magnitudes, it's much easier if the spectrum is expressed in terms of wavelength.

ngc3314
2011-Oct-18, 12:32 PM
It's simply a practical issue. Most passbands in the optical are described in terms of transmission vs. wavelength. So, if one wants to convolve some spectrum with those passbands to compute synthetic magnitudes, it's much easier if the spectrum is expressed in terms of wavelength.

And then there are those astronomers (some of them Quite Eminent) who made a habit of plotting spectra as F-nu versus wavelength. Where are the torch-waving villagers when you need them? Or is it too much of an idiosyncracy to want the integral under the curve to have an obvious meaning? (OTOH, I do see use for those nu*F-nu plots as long as it's versus frequency).

George
2011-Oct-18, 09:24 PM
Here is a nice summary, with lots of the key formulae and concepts bundled into a few short (PDF) pages: ASTROPHYSICAL INFORMATION DERIVED FROM THE EM SPECTRUM (http://www.astro.virginia.edu/class/oconnell/astr511/lec2-f03.pdf). That's helpful.

You're right, ngc3314, about the 1000 photon flux a zero mag. The link shows that, in georgeeze, if some punk -- in your inertial frame -- was shining a green laser at you, and its mag. was 0, then you would be receiving 1,005.1 green (550nm) photons per sec. per cm2. At least that's my interpretation of Zero Point monochromatic app. mag.

George
2011-Oct-18, 09:36 PM
And then there are those astronomers (some of them Quite Eminent) who made a habit of plotting spectra as F-nu versus wavelength. Where are the torch-waving villagers when you need them? Give me a sign; I'll march! I've seen those labels and they are salt on the wound because they don't make sense unless you do integrate -- so they need to use appropriate labeling. In some cases they don't even show a derivative form for the y-axis. Heliochromology almost died during its infancy because of this. Hmmmm, that might actually explain why they do that, I suppose. ;)

Nereid
2011-Oct-20, 07:25 PM
But first, a question: TvD11's Figure 6 y-axis is labelled "μλ [mag/arcsec2]"; however, SDSS ugriz magnitudes are AB magnitudes! Why μλ? Why not μν?



It's simply a practical issue. Most passbands in the optical are described in terms of transmission vs. wavelength. So, if one wants to convolve some spectrum with those passbands to compute synthetic magnitudes, it's much easier if the spectrum is expressed in terms of wavelength.


And then there are those astronomers (some of them Quite Eminent) who made a habit of plotting spectra as F-nu versus wavelength. Where are the torch-waving villagers when you need them? Or is it too much of an idiosyncracy to want the integral under the curve to have an obvious meaning? (OTOH, I do see use for those nu*F-nu plots as long as it's versus frequency).

Thanks StupendousMan, thanks ngc3314.

I'm still puzzled. Here's why (bear with me please; this is an interactive session, in which I try to write in [ t e x ]):

First, the standard definition:

\mu_\lambda \equiv m_\lambda + 2.5log_{10}\Omega

where m_\lambda is the integrated magnitude of the source and \Omega is the angular area of the source in units of arcsec2.

Presumably the following is also correct (is it?):

\mu_\nu \equiv m_\nu + 2.5log_{10}\Omega

where m_\nu is the integrated magnitude of the source and \Omega is the angular area of the source in units of arcsec2.

How do these relate to spectral flux density, f_\lambda or f_\nu?

Well, observations are made in bands (wavebands, passbands):

\langle f_\lambda\rangle = \int {T(\lambda)f_\lambda} {d\lambda}\ / \int{T(\lambda){d\lambda}

where T is the system response function. Now if T is perfect, then:

\langle f_\lambda\rangle = \int {f_\lambda}{d\lambda}

Presumably the following is also correct, for a perfect system response function (is it?):

\langle f_\nu\rangle = \int {f_\nu}{d\nu}

Almost there.

For a perfect band, the limits of integration are \lambda_1 and \lambda_2; they are also, for \langle f_\nu\rangle, \nu_1 and \nu_2.

And, if it's the same band: \nu_2 = c/\lambda_1, \nu_1 = c/\lambda_2.

Now a band is a band is a band; assuming perfection (including the definition of a particular band!), if m_V = 12.3 mags (say) for a particular star, does it make sense to write 12.3 _\lambda mags (or 12.3 _\nu mags)?

Nereid
2011-Oct-20, 10:04 PM
Thinking about this another way ...

The perfect filter allows all the wavelengths (frequencies) of EM radiation, within a specific range, through, perfectly, and none whatsoever outside that range. The perfect detector produces an output that is linear with respect to the total energy that comes through the filter (directly proportional, constant = 1). Situated above the Earth's atmosphere, oriented normal to the incoming wavefronts, with an area of 1 cm2, the system (filter plus detector) outputs values of energy per unit time.

In what way does it make sense to refer to these energies (per unit time, per ...) - as 'wavelength' mags (m_\lambda)? as 'frequency' mags (m_\nu)?

StupendousMan
2011-Oct-21, 02:19 AM
First, I don't understand the first two equations you wrote in message 81. The letter "m" is used for magnitude -- okay. The letter "mu", in astronomy, sometimes means surface brightness, in magnitudes per square arcsecond. The letter "Omega" can mean the surface area of an object .. well, maybe. But the three quantities aren't related by the equation that you gave, as far as I can tell, and I don't see why one would bring surface brightness into this discussion, anyway.

Moving on, I don't think that it makes sense to talk about "wavelength" magnitudes or "frequency" magnitudes. We collect energy or photons from a distant star, and measure the amount of energy per unit area per unit time. We then assign a number called the magnitude to each star, based on how much energy we receive. Once you choose some particular passband, it doesn't matter whether you describe it in terms of frequency or in terms of wavelength: the amount of energy which is collected from a given star through that passband is the same.

I get the feeling that I'm missing the thrust of your questions. If you ask in a different way, maybe I'd be able to understand the questions better. Sorry :-/

Nereid
2011-Oct-21, 07:27 AM
First, I don't understand the first two equations you wrote in message 81. The letter "m" is used for magnitude -- okay. The letter "mu", in astronomy, sometimes means surface brightness, in magnitudes per square arcsecond. The letter "Omega" can mean the surface area of an object .. well, maybe. But the three quantities aren't related by the equation that you gave, as far as I can tell, and I don't see why one would bring surface brightness into this discussion, anyway.
Thanks! :)

My post started out as one thing, but quickly became all about trying to get [ t e x ] tags to work properly.

What I am working towards is my own answer to an earlier question, here re-phrased using tex tags:

TvD11's Figure 6 y-axis is labelled " \mu_\lambda \ [mag/arcsec^2] "; however, SDSS ugriz magnitudes are AB magnitudes! Why \mu_\lambda? Why not \mu_\nu?

The first equation, in my earlier post, comes from the source I linked to earlier, the University of Virginia's "ASTR 511/OĺConnell Lec 2"; in hindsight I should have *quoted*, and provided a source (the only parts I omitted are the heading - "Surface Brightnesses (of extended objects):" - this, at the end of the first bullet: "1 arcsec2 = 2.35 X 10-11 steradians", and the second bullet [1])

Whatever; the O'Connell document seems to have left out (i.e. implied) this: "at λ" (i.e. the definition of the surface brightness of an extended object, at a wavelength of λ, is ...).


Moving on, I don't think that it makes sense to talk about "wavelength" magnitudes or "frequency" magnitudes. We collect energy or photons from a distant star, and measure the amount of energy per unit area per unit time. We then assign a number called the magnitude to each star, based on how much energy we receive. Once you choose some particular passband, it doesn't matter whether you describe it in terms of frequency or in terms of wavelength: the amount of energy which is collected from a given star through that passband is the same.
Yes, that's the feeling I had earlier ... but the axis-label in Tal & van Dokkum's paper's is as I've quoted it, and the O'Connell document uses the same term.

Most of the rest of my post is about showing that, just as you write, it doesn't make sense to talk about λ magnitudes or ν magnitudes, if you're working with bands [2]. So, the λ subscript in TvD11's Figure 6 is irrelevant.


I get the feeling that I'm missing the thrust of your questions. If you ask in a different way, maybe I'd be able to understand the questions better. Sorry :-/
I hope it's clearer now ...

[1] The second bullet reads:
μ is the magnitude corresponding to the mean flux in one arcsec2 of the source. Units of μ are quoted, misleadingly, as ômagnitudes per square arcsecond.ö
[2] Clearly specifying, or defining, the magnitude system is important of course!

ngc3314
2011-Oct-21, 12:22 PM
As I think about it, magnitudes would be usually defined only for a particular band (UBVRI, ugriz, whatever), and then the wavelength-frequency distinction would be mot - the band is what it is. The only way I can think of for m-lambda to make sense would be a sort of AB magnitude analog where data at different wavelengths are being combined-compared (such as because of redshift changes). There is a common UV magnitude system which is defined like m(UV)=constant - 2.5*log(F-lambda) across wavelengths (although one must still specify where it's measured for useful comparisons).

StupendousMan
2011-Oct-21, 12:42 PM
TvD11's Figure 6 y-axis is labelled " \mu_\lambda \ [mag/arcsec^2] "; however, SDSS ugriz magnitudes are AB magnitudes! Why \mu_\lambda? Why not \mu_\nu?


I've just scanned the TvD11 paper, and can't find any explanation for the y-axis label in their Figure 6. I can only speculate that they mean to say, "the quantity plotted on this axis represents the surface brightness in u-band magnitudes per square arcsecond for the u-band points on the graph, and surface brightness in g-band magnitudes per square arcsecond for the g-band points, etc."

If I were reviewing the paper, I'd ask them to change that label or at least try to explain what they mean.

Nereid
2011-Oct-22, 01:48 PM
Thanks ngc3314, StupendousMan.

The little excursion into understanding flux, intensity, magnitudes etc may have seemed a but of a diversion, a luxury, even irrelevant.

However, for me it was not. It allowed me to discover an error, an approximation too far, (or something similar) in the D&L paper.

At the bottom of page 10, top of page 11 (may not work perfectly, tex tags inside a quote, etc):

To keep things simple we consider only exponential galaxies and ignore Tolman dimming and cosmology for now (see later). If we adopt de Vaucouleurs (1959) 2-parameter intensity I(r) profiles for galaxies, i.e.
ln\frac{I(\theta)}{I(0)} = -\left(\frac{\theta}{\alpha}\right)^{\frac{1}{\beta }
But this is not correct; the "2-parameter intensity I(r) profiles for galaxies" are band-specific!

So, more accurately, for the V-band there's:
ln\frac{I_V(\theta)}{I_V(0)} = -\left(\frac{\theta}{\alpha_V}\right)^{\frac{1}{\be ta_V}

for the B-band there's:

(you get the idea).

Does this matter? Well, yes, it does! Why? Stay tuned! :)

ngc3314
2011-Oct-22, 05:50 PM
Expanding a bit on your last point, most kinds of galaxies have color gradients, either as a result of differing star-formation histories or changes in metal abundance. At their most extreme, they can make a galaxy look completely different between ultraviolet, optical, and near-infrared bands, something known in the trade as the morphological k-correction. An example is shown in this series depicting M81 (http://www.astr.ua.edu/gifimages/m81series2.html) across the spectrum. In the UV, the central bulge nearly vanishes, while in the near-IR we barely see the star-forming regions in the arms. At the least, this means that different light-distribution laws would be needed to model the detectability of such galaxies at low and high redshifts, unless one had data at closely matching emitted wavelengths (which accounts for the attention given to doing the Hubble Deep Fields in the near-IR, and the near-IR images of the giant CANDELS Hubble project).

Nereid
2011-Oct-23, 11:04 PM
Expanding a bit on your last point, most kinds of galaxies have color gradients, either as a result of differing star-formation histories or changes in metal abundance. At their most extreme, they can make a galaxy look completely different between ultraviolet, optical, and near-infrared bands, something known in the trade as the morphological k-correction. An example is shown in this series depicting M81 (http://www.astr.ua.edu/gifimages/m81series2.html) across the spectrum. In the UV, the central bulge nearly vanishes, while in the near-IR we barely see the star-forming regions in the arms. At the least, this means that different light-distribution laws would be needed to model the detectability of such galaxies at low and high redshifts, unless one had data at closely matching emitted wavelengths (which accounts for the attention given to doing the Hubble Deep Fields in the near-IR, and the near-IR images of the giant CANDELS Hubble project).
A somewhat more detailed examination of these points may be found here (http://www.astr.ua.edu/keel/galaxies/components.html) (the author is a certain Bill Keel!), and here (http://www.ifa.hawaii.edu/~barnes/ast626_97/mc.html).

In addition to this, consider mergers. The first Galaxy Zoo paper (DR1 (http://arxiv.org/abs/1007.3265), link to follow) found that 'mergers' constitute a minor, but not totally insignificant, fraction of SDSS 'galaxies' with z < 0.25.

The key points, so far as the D&L paper is concerned, would seem to be:
-> it's not just "Giant Ellipticals" which have a radial intensity profile closely approximated by β=4, but (local) ellipticals in general, and the bulges of (local) spiral galaxies (for bands such as B, V, g, r)
-> the observed profile for (local) spiral disks has a projection component, and edge-on disks have a rather different profile (again, for visual/optical bands)
-> "indeed many galactic disks are seen to deviate from (2) [the exponential profile] at smaller radii" (from the second source)
-> perhaps the most important - empirical - radial profiles to start with are those of local galaxies (as determined by their redshifts) in various UV bands, including those which have the rest Lyman-α (and higher) within them.

(more later)

Nereid
2011-Oct-24, 10:54 PM
Section IV ("IMPRISONED BY LIGHT") looks, at skim-level, pretty impressive, and its conclusion* inescapable.

However, this whole section uses the material developed earlier, without any explicit recognition of its assumptions! This may - I haven't done enough work on it yet to be sure - prove fatal to its stated conclusions. In particular, the band-specific assumption seems to have been ignored, and the explicit "we consider only exponential galaxies and ignore Tolman dimming and cosmology for now" assumptions not revisited/restated/added-in/etc! :eek:

This is particularly ironic, as the very next section (V "HOW GALAXIES SINK FROM SIGHT") starts with these words (bold added):

The Visibility Window depicted in fig 3 is immutable, mathematical and pinned in local coordinates because it shows the contrast to ones local sky, be it on the ground or in space. What we need to calculate next are the properties, in particular the sizes and intrinsic SBs, of the kinds of galaxies, seen at different redshifts, which will make it through that narrow window, particularly near its peak, taking into account the Tolman effects described above, which both dim a galaxy and increase its apparent size.

Can you spot the - potentially fatal - inconsistency?

Disney & Lang calculate "The Visibility Window depicted in fig 3" assuming "only exponential galaxies and ignore Tolman dimming and cosmology" (and that the intensity profiles - especially the exponential one - are universal, and not band-specific). Then, assuming this result "is immutable" [sic!], they then proceed to take "into account the Tolman effects". To be quite clear, I haven't (yet) gone through their subsequent work; however, at first blush this looks like a fatal logical flaw. A more rigorous - and convincing - approach would be to re-work the earlier sections, starting with the more general (band-specific) intensity profile and Tolman dimming and cosmology.

(to be continued)

* "For all practical purposes then we are implacably imprisoned in our cell of light. Classes of low SB galaxies unresolved into stars, which cannot already be seen in Schmidt surveys, are beyond hope of discovery by optical means alone. It follows that large hidden populations of low surface brightness galaxies, both near and far, cannot be ruled out by optical observations alone. This is a much stronger statement than could have been made before and it relies on the arguments which led to eqn.(26)"

Jerry
2011-Oct-25, 07:46 AM
Can you spot the - potentially fatal - inconsistency?

Disney & Lang calculate "The Visibility Window depicted in fig 3" assuming "only exponential galaxies and ignore Tolman dimming and cosmology" (and that the intensity profiles - especially the exponential one - are universal, and not band-specific). Then, assuming this result "is immutable" [sic!], they then proceed to take "into account the Tolman effects". To be quite clear, I haven't (yet) gone through their subsequent work; however, at first blush this looks like a fatal logical flaw. A more rigorous - and convincing - approach would be to re-work the earlier sections, starting with the more general (band-specific) intensity profile and Tolman dimming and cosmology.
I don't see the problem: Tolman effects - if they are real, always reduce the visibility window; and should not enhance it in any spectral range.

Nereid
2011-Oct-25, 01:22 PM
Some bits and pieces ...

Expanding a bit on your last point, most kinds of galaxies have color gradients, either as a result of differing star-formation histories or changes in metal abundance. At their most extreme, they can make a galaxy look completely different between ultraviolet, optical, and near-infrared bands, something known in the trade as the morphological k-correction. An example is shown in this series depicting M81 (http://www.astr.ua.edu/gifimages/m81series2.html) across the spectrum. In the UV, the central bulge nearly vanishes, while in the near-IR we barely see the star-forming regions in the arms. At the least, this means that different light-distribution laws would be needed to model the detectability of such galaxies at low and high redshifts, unless one had data at closely matching emitted wavelengths (which accounts for the attention given to doing the Hubble Deep Fields in the near-IR, and the near-IR images of the giant CANDELS Hubble project).
Here (http://spaceplace.nasa.gov/galaxy-montage2/) is a montage of GALEX images (in the UV, obviously), alongside ones taken in "visible light" (and presented at the same scale). This makes the same point, and more: while spirals may, on average, have exponential profiles, in the V-band, quite a few clearly do not, in UV-bands. [1]

There's another aspect that may be important to keep in mind; here's Disney & Lang (bold added):

Classes of low SB galaxies unresolved into stars, which cannot already be seen in Schmidt surveys, are beyond hope of discovery by optical means alone.

SDSS images of Local Group galaxies show that the uncrowded parts are easily resolved into stars; peering a bit further out and you come across objects like SDSS J143759.94+400622.3 (http://cas.sdss.org/astro/en/tools/explore/obj.asp?id=588017627240333559) (check out this SDSS image (http://casjobs.sdss.org/ImgCutoutDR6/getjpeg.aspx?ra=219.4997522&dec=40.10619393&scale=0.19806&width=512&height=512&opt=&query=) of it) and NGC 4395 (http://casjobs.sdss.org/ImgCutoutDR7/getjpeg.aspx?ra=186.454&dec=33.547&scale=1.58448&opt=&width=512&height=512). The bright blue blobs are star-forming regions, perhaps like the Tarantula Nebula (in the LMC) or NGC 604 (in M33). While the underlying intensity profile in galaxies with lots of these sorts of regions may be exponential, what stands out - especially in bands which include the strong emission lines from such regions - is the blobs. The Visibility (to use the D&L term) of these kinds of galaxies is surely determined by the number, spacing, and surface brightness of the blobs, not the underlying galaxy! [2]

[1] of course, a quantitative analysis would be needed, to show that the radial profile differs markedly from an exponential one, for these galaxies at least.
[2] a point already made by ngc3314 (http://www.bautforum.com/showthread.php/120972-Bias-effects-in-galaxy-detection?p=1934869#post1934869), early on page 1 of this thread:

[...] higher resolution lets us see the bright cores and star-forming regions, while redshifting ultraviolet into the visible band would exacerbate light loss due to dust while presenting us brighter and more compact star-forming regions when they are not heavily dust-reddened. One clear result of the Tolman dimming is that our optcally-chosen galaxy samples at high redshifts cannot fail to be biased in favor of galaxies or pieces of galaxies which are blue (UV-bright) of unusually high surface brightness.
And what does this have to do with low SB galaxies? Not much - :p - except to stress, again, that D&L's analysis has some limits (possibly quite severe ones)

Nereid
2011-Oct-25, 01:45 PM
I don't see the problem: Tolman effects - if they are real, always reduce the visibility window; and should not enhance it in any spectral range.
A rather odd comment, especially coming from you, Jerry!

What part of "band-specific" did (do) you not understand? of "the morphological k-correction"?

Nereid
2011-Oct-25, 03:16 PM
Second para of Disney & Lang's Section V. ("HOW GALAXIES SINK FROM SIGHT"):

The (1 + z)^{-4} factor rapidly becomes very significant by comparison with the narrow FWHM (2.5 mag) of the Visibility Window. Even at z = 0.5 many of the most Visible galaxies that were in region A (Fig 1) at low redshift would be translated into region C and be far too dim to see.
Except that, of course, at z = 0.5, what is the U-band [1] at z ~0 is observed in the V-band (more or less); and the V-band (at z~0) is now some non-standard band, neither R- nor I-band.

Using the terms D&L introduce in Section III ("AN OUTLINE OF VISIBILITY THEORY"), \alpha, \beta, and N are all band-specific! This renders the rest of their argument moot ... unless they can show that these key parameters have the same values, irrespective of band. Of course they assume \beta=1, which we already know to be, in general, an invalid assumption. [2]

Time to introduce J.L. SÚrsic. From the Keel document I linked to earlier, the SÚrsic profile:

I(r)=I(0)exp(-b_n(r/r_e)^{1/n}),

where re is the effective radius.

Putting this into the form in D&L:

ln(I(r)/{I(0)})=-b_n(r/r_e)^{1/n}

Which is the same as D&L's (2), where (I hope!):
\theta=r,
\beta=n, and
\alpha=r_e{b_n}^{-n}

To illustrate the band-specific nature of the intensity profile, consider the best-fit parameters in TvD11, to the stacked bright red ellipticals; from Table 1 ("Stack SÚrsic parameters"); the columns are, respectively Filter, SÚrsic index, and Effective radius (kpc):
u 3.94▒1.62 17.0▒9.80
g 4.03▒0.09 12.6▒0.19
r 5.50▒0.05 13.1▒0.10
i 4.86▒0.05 10.9▒0.06
z 4.91▒0.08 11.5▒0.12

Recall that D&L assume a SÚrsic index of 1, and a constant (band-independent) α; but note too that Section V explicitly does not cover this type of galaxy (that's left to Section VIII "HOW ELLIPTICAL GALAXIES SINK").

Incidentally, by ignoring the band-specific nature of the intensity profiles (including the fact that 'pure exponentials' may be the majority of galaxies in only a narrow wavelength range!), D&L's Section VI ("WHY HIGH REDSHIFT GALAXIES LOOK SMALL") and Section VII ("WHERE HAVE THE DESCENDANTS GONE?") become essentially moot too.

What - other than re-doing the main arguments in D&L - is left to consider then?

[1] Johnson-Morgan photometric system; ditto V-band, R-band, I-band
[2] Get it now Jerry?

Nereid
2011-Oct-27, 07:12 AM
The Galaxy Zoo forum (http://www.galaxyzooforum.org/index.php) is a terrific place to hang out, and feast your eyes on some interesting images (and discussions).

I expect that it could provide plenty of examples (or counter-examples) relevant to this thread. Not in any quantitative, survey-level sense, but as pointers to what it might be interesting to look for in the outputs from SDSS and COSMOS.

The results of the first Galaxy Zoo project - i.e. as described in the Lintott et al. paper I provided a link to earlier - are incorporated in DR8 (http://skyserver.sdss3.org/dr8/en/help/docs/tabledesc.asp), which is very useful. :clap:

Oh, and the current Galaxy Zoo project (http://www.galaxyzoo.org/) a very cool one to participate in, involving classifying images from the Hubble, from some of the big surveys done with the HST (e.g. COSMOS (http://arxiv.org/abs/astro-ph/0612305)). :)

Nereid
2011-Oct-27, 03:32 PM
As anyone who has looked for M31 can testify, the problem of detecting galaxies in the optical is not so much lack of light as lack of contrast against the foreground sky[M31 has a V mag of 3.4 which spread over its size of roughly 3 by 1 degrees amounts to a SB = 21.2 V mag per sq arc sec , where the sky is about 21.5 at a fair site]
That's how Section II ("THE NARROW WINDOW"), the start of the make part of the D&L paper, begins.


FIG. 3: The calculated Visibility Window for Exponential galaxies. The vertical scale shows the relative volumes within which galaxies with different surface-brightness-contrasts to the background (plotted horizontally) can be detected. Following the usual convention this contrast Δμ, in mag, is plotted from right to left with high surface brightness, i.e. high contrast galaxies to the left, low surface brightness galaxies to the right. The maximum heights of the two curves Vm (dashed or green) and Vθ (smooth or red) assume a sample for which Γc = π , typical of all Exponential galaxies, save those hundreds of pixels across. This is a typical Wigwam Diagram for the Visibility of galaxies of all kinds ( see later). Since the vertical scale is arbitrary the Wigwam Diagram is valid irrespective of Absolute Luminosity, just as it is valid irrespective of the absolute survey depth ( deepest isophotal level μc ) because the horizontal axis is given only in contrast Δμ ≡ (μc-μ0) where the latter is the central SB, measured in magnitudes. To be detected galaxies must lie inside the Wigwam, the shaded area marked A, which we call The Visibility Window. Note how narrow it is, with a FWHM of 2.5 magnitudes with a peak P at a contrast of 3.5 magnitudes .Because the Window is so narrow, redshift dimming will quickly move galaxies rightward and out of sight into regions C and even D. For future reference note that even the Vθ(smooth or red) curve, by itself, has a FWHM of only 3 magnitudes.
Now I get 22.37 V mag per sq arc sec as the SB for M31, but that's close enough, given the "roughly".

Of course, M31 is "hundreds of pixels across" :p, so the text for Fig 3 doesn't apply; however, if it did, what would Δμ be? To answer that, we need to know μ0 (we already know that μc is ~21-22).

Would any reader like to have a go at working this out?

FWIW, a blind guess by me would be μ0 ~ 5, giving a Δμ > 15, making M31 an extreme outlier! :D

Now suppose we took an image of M31, with a modern CCD-based SLR camera, attached to an equatorial mount (so the image won't smear during the shot), at a zoom that ensures M31 will be no more than ~100 pixels across. What would Δμ be?

Nereid
2011-Oct-29, 12:30 PM
That's how Section II ("THE NARROW WINDOW"), the start of the make part of the D&L paper, begins.


Now I get 22.37 V mag per sq arc sec as the SB for M31, but that's close enough, given the "roughly".

Of course, M31 is "hundreds of pixels across" :p, so the text for Fig 3 doesn't apply; however, if it did, what would Δμ be? To answer that, we need to know μ0 (we already know that μc is ~21-22).

Would any reader like to have a go at working this out?

FWIW, a blind guess by me would be μ0 ~ 5, giving a Δμ > 15, making M31 an extreme outlier! :D

Now suppose we took an image of M31, with a modern CCD-based SLR camera, attached to an equatorial mount (so the image won't smear during the shot), at a zoom that ensures M31 will be no more than ~100 pixels across. What would Δμ be?
No surprise to learn that BAUT's Astrophotography section contains images of M31, or links to such. And they cover a wide range of equipment and conditions, including several much like the above.

For example, this by Moonhawk (http://www.bautforum.com/showthread.php/98553-Orion-and-Andromeda-5D-mkII?highlight=M31+andromeda). The downside, of course, is that the images on the web are JPEGs, and they do not preserve flux. Maybe, however, the authors still have their original - digital - files, and might be willing to share them?

Jerry
2011-Nov-01, 08:29 AM
Second para of Disney & Lang's Section V. ("HOW GALAXIES SINK FROM SIGHT"):

Except that, of course, at z = 0.5, what is the U-band [1] at z ~0 is observed in the V-band (more or less); and the V-band (at z~0) is now some non-standard band, neither R- nor I-band...


[1] Johnson-Morgan photometric system; ditto V-band, R-band, I-band
[2] Get it now Jerry?

Yes, thank you. When the redshift moves intense U-band into visible ranges, we get bonus observables...but not after transforming the spectrum back to the reference frame - isn't this where the comparison between galaxy morphologies becomes important?

Nereid
2012-Jan-14, 05:56 AM
Astrophysically Motivated Bulge-Disk Decompositions of SDSS Galaxies (http://arxiv.org/abs/1201.0763), by C. N. Lackner and J. E. Gunn, makes an extremely good case for saying that characterising galaxies as either exponential or de Vaucouleurs (in terms of their surface brightness distributions) is too simplistic by far. And it's a great paper to read anyway.

A point on Tolman: isn't one of the four (1+z) factors just 'band shifting'? I.e. the 'surface brightness' is the total (apparent) energy per unit area (on the sky), and one of the (1+z) factors comes from the fact that the whole SED (appears to) moves redward? If this is so, then Disney and Lang's argument is even more flawed than I'd thought (they should be using (1+z)^3, as all observations of (optical) galaxies are made within specific (visual) wavebands).

Or do I misunderstand the Tolman surface brightness concept?

StupendousMan
2012-Jan-14, 07:59 PM
[Edit: the explanation below mistakenly derives a (1+z)^4 law for extended sources. The derivation is actually a correct derivation for POINT sources. To see the proper derivation for extended sources, look in post 103 below]

The four (1+z) factors in the Tolman effect explain why the surface brightness of a distant galaxy -- as measured in flux units, energy per unit area per unit time per unit solid angle -- decreases so quickly.

1 and 2. the inverse square law, decreasing flux with distance in the usual manner
3. time dilation: clocks on distant objects appear to run slowly, and so distant objects appear to emit fewer photons each second
4. decrease in energy of each photon due to the redshift of each photon

Note that this effect deals with measurements of energy flux: how many ergs per square cm per second are received.

In this context, people ignore the change in received flux due to the shifting of the object's spectrum through the detector's passband. That effect is very real, of course, but bringing it up opens a whole new kettle of fish. The discussion about (1+z)^4 centers on the properties of space and time, not properties of the source. In other words, people who talk about (1+z)^4 are interested in cosmology, not in the evolution of stars and galaxies.

There ARE plenty of people who do care deeply about the properties of galaxies and their evolution, of course. They will include in great gory detail the effects of the shifting of the source spectrum through the detector passband in these discussions.

One of the original references is Hubble and Tolman, "Two Methods of Investigating the Nature of the Nebular Redshift", ApJ 82, 302 (1935).

http://adsabs.harvard.edu/abs/1935ApJ....82..302H

Nereid
2012-Jan-17, 07:55 AM
Thanks SM! :)

Now I'm rather puzzled and confused.

Disney has been doing astronomy for decades, full-time, and many of the published papers of which he an author concern observations of galaxies, in various wavebands. He's also written on cosmology (I have no idea about the co-author, R.H. Lang).

So for him to have written a paper (well, it's only a draft at this stage) whose main thrust is based on a fatal misunderstanding of the Tolman surface brightness relationship is, well, pretty astonishing, isn't it?

But more puzzling is that it seems he (Disney) has been working on this 'problem' for quite some time (e.g. "Some of these ideas were explored in 'The Visibility of High Redshift Galaxies' ( Phillipps, Davies & Disney 1990) which built on earlier papers in 1983 (Disney & Phillipps) and 1976 (Disney). However the highest redshift being considered there and then was 0.3!"); how could he have messed up so badly? I mean, if instead of being a (tenured?) professor, he was one of your (graduate) students, I doubt you'd've given him a pass if this were a term assignment, would you?

antoniseb
2012-Jan-17, 10:49 AM
The four (1+z) factors in the Tolman effect explain why the surface brightness of a distant galaxy -- as measured in flux units, energy per unit area per unit time per unit solid angle -- decreases so quickly.

1 and 2. the inverse square law, decreasing flux with distance in the usual manner
3. time dilation: clocks on distant objects appear to run slowly, and so distant objects appear to emit fewer photons each second
4. decrease in energy of each photon due to the redshift of each photon
...

I'm late coming to this discussion, but I have a question about the above... I *easily* see how (1+z)^4 applies to stars and supernovae, but wouldn't it be (1+z)^2 for a source extended larger than a pixel (galaxies) in the receiving imager?

StupendousMan
2012-Jan-17, 01:24 PM
Whoops. Antonsieb has it right: the apparent brightness of a point source will decrease as (1+z)^4. The short explanation I gave was, in fact, not appropriate for extended sources, but for point sources. It was my mistake -- I study supernovae, not galaxies, so I'm always thinking about those four factors.

Strangely enough, the apparent brightness of an extended source will ALSO decrease as (1+z)^4 in an expanding universe! The reasons are a little different. For an extended source, the simple inverse-law decrease in apparent brightness is countered by the increase in the source luminosity falling into the measurement aperture; that is, if we measure the amount of light in a circle of radius 3 arcseconds, then if a galaxy moves twice as far away, each little bit provides only 1/4 as much light, but at this larger distance, 4 times as many stars fall into our circle of 3 arcseconds. So, in a static universe, the surface brightness of an extended source would be constant.

In an expanding universe, the items 3 and 4 I listed in my original message, the time dilation and redshifting of each photon, would still apply. That would cause the surface brightness of a galaxy to decrease by a factor of (1+z)^2. But wait -- there's more! In an expanding universe, the apparent angular size of a galaxy does NOT decrease exactly as it does in a static universe. Instead of constantly shrinking and shrinking with distance, a galaxy will, at high redshift, start to increase in apparent angular size. It's hard to visualize (at least for me), but the basic idea is that when the galaxy emitted its light, it was much closer to us, which makes it apparent size (back then) larger than we'd otherwise expect.

The bottom line is that this increase in apparent angular size with redshift adds another factor of (1+z)^2 to the equation. The apparent surface brightness (measured by energy) of extended objects in an expanding universe decreases as (1+z)^4, AND the apparent brightness (measured by energy) of point sources also decreases as (1+z)^4. Kind of neat, now that I write them both.

These simple calculations assume that the spectrum of the source object doesn't change over the range of observed wavelengths, regardless of the redshift. In real life, the energy emitted by source objects can change quite sharply over the wavelengths sampled in, say, the observer's V-band as redshift changes from z=0 to z=7; so in real life, comparing observations to theory is much more difficult.

ngc3314
2012-Jan-17, 02:50 PM
Wait a minute! Something cannot be right here, because in "point sources" we are integrating over the entire angular extent so that cosmological departures from inverse-square (heuristically due to changes in apparent size between emission and observation) don't matter. In common jargon, this relates to the difference between luminosity distance and angular-diameter distance. All this is easiest to work with for bolometric fluxes - there one has to deal with (1+z) factors for photon arrival rate and energy, but not for narrowing or shifting of the filter bands in the emitted frame.

Nereid
2012-Jan-17, 03:26 PM
Earlier in this thread one of the Lubin and Sandage (2001) papers, on an observational test of the Tolman surface brightness test, was referenced. There are altogether four in the series. The first begins with this (some formatting is lost):

Tolman (1930, 1934) derived the remarkable result that, in an expanding universe with any arbitrary geometry, the surface brightness of a set of "standard" (identical) objects will decrease by (1+z)^4. One factor of (1+z) comes from the decrease in the energy of each photon due to the redshift. The second factor comes from the decrease in the number flux per unit time. Two additional factors of (1+z) come from the apparent increase of area due to aberration. The effect is the same for all intrinsic geometries because the cosmological geometric effects due to different space curvatures (i.e., the dependence on q0) are identical in the equations for luminosity, L =f (q0,z) and intrinsic radius= g(q0,z). Hence, the ratio of L to (radius)^2, which is the surface brightness, is independent of all the cosmological parameters, precisely (Sandage 1961, 1972).

The critical point - that underlies Disney&Lang's apparent fatal flaw - is that the key dimension of "luminosity" is energy (per unit time, in the observer's frame).

This luminosity is band-independent; it is a total energy, integrated over all wavelengths (frequencies) of light (electromagnetic radiation).

Disney&Lang, however, conflate the luminosity of the Tolman surface brightness test with the luminosity of galaxies in a very, very narrow slice of the electromagnetic spectrum (basically, little more than 'the visual', from the atmospheric UV cutoff to ~1 micron), e.g. "Fig 1 illustrates what happens for objects with an exponentially declining light distribution ( virtually all galaxies bar Giant Ellipticals; see later)."

NGC3314 mentioned "k-corrections", which is highly relevant; Disney&Lang use the phrase precisely once, on p26, and promptly proceed to ignore it ("Given uncertainties as to which is the correct model, and K-corrections, dust and Evolution, this approximation is more than satisfactory").

As I currently understand it, Disney&Lang say - among other things - that the surface brightness of a galaxy like the Milky Way will drop by (1+z)^4 when observed in a band which shifts with z (as such a galaxy is 'placed' some distance away from us); if we use V as our band to perform observations, then as our test galaxy gets further and further away, the band in which we observe it becomes shifted further and further into the red. Here's where their conflating of the two different definitions of luminosity causes their fatal flaw: redshifting is, itself, one of the (1+z) factors!

In addition, as I pointed out earlier, the sky is band-dependent; the limiting surface brightness - due to the sky - is not the same in all bands. So, the extent to which a galaxy's surface brightness falls below the sky, as its apparent surface brightness declines (by a factor of (1+z)^3, in a band which tracks z), depends critically on what the surface brightness of the sky is, in the two different bands.

StupendousMan
2012-Jan-17, 06:07 PM
I wrote: measured bolometric flux of point source goes like (1+z)^4. Measured bolometric flux of extended source goes like (1+z)^4 also, but for different reasons.


Wait a minute! Something cannot be right here, because in "point sources" we are integrating over the entire angular extent so that cosmological departures from inverse-square (heuristically due to changes in apparent size between emission and observation) don't matter. In common jargon, this relates to the difference between luminosity distance and angular-diameter distance. All this is easiest to work with for bolometric fluxes - there one has to deal with (1+z) factors for photon arrival rate and energy, but not for narrowing or shifting of the filter bands in the emitted frame.

Perhaps our disagreement is due to the (many) different ways one can define "brightness". Let's say that a standard 100 Watt light bulb is observed at many different distances, at many different values of "z". It acts like a point source for our telescopes on Earth. How does observed bolometric flux (ergs per sq. cm. per second, integrated over all wavelengths) behave as a function of z, in a standard expanding universe, for values of 0.01 < z < 1?

ngc3314
2012-Jan-17, 07:01 PM
Indeed, the spatially integrated bolometric flux of point and extended sources must have the same behavior. But for extended sources, it's more usual (and germane to galaxy detection) to deal with surface brightness, at some reference radius or within some metric radius. For each, spatially integrated bolometric flux does the same thing. For some specific numbers, I took Ned Wright's cosmology calculator (http://www.astro.ucla.edu/~wright/CosmoCalc.html) with "consensus" WMAP parameters plus flat geometry, and consider an object with bolometric magnitude Mbol=-24. The apparent bolometric magnitude mbol is just -24 + 5 log (D/10 pc). To show surface brightness, let this flux be uniformly distributed in a circular disk with radius 10 kpc (thus over an area pi*(10/angular scale)^2 arcsec^2). The table lists z, luminosity distance, angular scale in kpc/arcsecond, mbol, and bolometric surface brightness in mag/arcsec^2.

z DL,Mpc ang scale, kpc/"" mbol Bolometric SB
0.01 42.6 0.20 9.14 18.88
0.1 455 1.82 14.29 19.23
0.25 1249 3.87 16.48 19.78
0.5 2822 6.08 18.25 20.57
0.75 4638 7.34 19.33 21.24
1.0 6634 8.04 20.10 21.82
1.5 11008 8.54 21.20 22.78
2.0 15733 8.47 21.98 23.56

(Rats. Where's a proportional font when you need it?)

At low z there is the familiar Euclidean limit - inverse-square for flux, constant surface brightness. But things get rapidly worse at cosmologically important distances, leading to almost a 5-magnitude drop in SB by z=2.0 (the Tolman signal would be 3^4=81 times or 4.77 magnitudes, a good match to the calculated 23.56-18.88=4.68 from the table, noting that the top point is not at z=0).

Nereid
2012-Jan-17, 07:41 PM
Consider an 'exponential' galaxy. Viewed locally (within the Local Group say, but from the outside), in the V band, its effective radius (Re) is 1 kpc. Assume the 1 kpc, at the distance we've observing it, is the same size as the seeing. Assume that, like SDSS, we can measure/detect the galaxy out to 3Re (in 2D), and truncate it to zero at 4Re. Let A be the brightness of the central, 1Re, point.

In a flat, Euclidean universe, when the galaxy is far enough away that 3Re corresponds to the seeing, what is the brightness of this (almost) point source (compared with A)?

In our universe, if the galaxy has a redshift of z, then one of the (1+z) effects is that we should observe the galaxy in a V band redshifted by z. If this were the only effect, then the galaxy would appear,when viewed in the redshifted V band, to be just like the same galaxy in a flat universe (observed in the V band).

The aberration, which comprises two of the (1+z) factors: compared with the same galaxy in a flat universe, the galaxy looks bigger; specifically, the seeing corresponds to 3/(1+z) Re, and the galaxy is clearly not a point source (assuming the redshifted V band has the same sky limit; i.e. we can measure/detect to 3Re, and truncate to zero at 4Re). But wait, can we still measure/detect out to 3Re? The 'fuzz' around the point souce that is now our galaxy - from 3/(1+z) Re to 3Re - has a lower surface brightness than that part of the galaxy when it was nearby (correcting for redshifting the V band).

Now add the last (1+z) factor; this simply makes every pixel dimmer.

If the logic is OK, it's time for some numbers ...

parejkoj
2012-Jan-17, 08:24 PM
(Rats. Where's a proportional font when you need it?)


I've got your proportional font right here!
123456789
abcdefghijkl
............

Now what're you gonna do about it?

...

Just put the text within FONT=Courier New ... /FONT tags.

Nereid
2012-Jan-19, 08:10 AM
Indeed, the spatially integrated bolometric flux of point and extended sources must have the same behavior. But for extended sources, it's more usual (and germane to galaxy detection) to deal with surface brightness, at some reference radius or within some metric radius. For each, spatially integrated bolometric flux does the same thing. For some specific numbers, I took Ned Wright's cosmology calculator (http://www.astro.ucla.edu/~wright/CosmoCalc.html) with "consensus" WMAP parameters plus flat geometry, and consider an object with bolometric magnitude Mbol=-24. The apparent bolometric magnitude mbol is just -24 + 5 log (D/10 pc). To show surface brightness, let this flux be uniformly distributed in a circular disk with radius 10 kpc (thus over an area pi*(10/angular scale)^2 arcsec^2). The table lists z, luminosity distance, angular scale in kpc/arcsecond, mbol, and bolometric surface brightness in mag/arcsec^2.

z DL,Mpc ang scale, kpc/"" mbol Bolometric SB
0.01 42.6 0.20 9.14 18.88
0.1 455 1.82 14.29 19.23
0.25 1249 3.87 16.48 19.78
0.5 2822 6.08 18.25 20.57
0.75 4638 7.34 19.33 21.24
1.0 6634 8.04 20.10 21.82
1.5 11008 8.54 21.20 22.78
2.0 15733 8.47 21.98 23.56

(Rats. Where's a proportional font when you need it?)

At low z there is the familiar Euclidean limit - inverse-square for flux, constant surface brightness. But things get rapidly worse at cosmologically important distances, leading to almost a 5-magnitude drop in SB by z=2.0 (the Tolman signal would be 3^4=81 times or 4.77 magnitudes, a good match to the calculated 23.56-18.88=4.68 from the table, noting that the top point is not at z=0).
I'm having difficulty accessing BAUT, so my last post crossed ngc3314's. Anyway, I think the approach outlined in it is better than the one in mine.

Some sanity checks:

z DL,Mpc ang scale, kpc/"" mbol Bolometric SB
0.01 42.6 0.20 9.14 18.88
0.1 455 1.82 14.29 19.23
0.25 1249 3.87 16.48 19.78
0.5 2822 6.08 18.25 20.57
0.75 4638 7.34 19.33 21.24
1.0 6634 8.04 20.10 21.82
1.5 11008 8.54 21.20 22.78
2.0 15733 8.47 21.98 23.56

In a flat, Euclidian ("flat") universe, the last column (bolometric SB) would be:
18.81
18.81
18.81
18.81
18.81
18.81
18.81
18.81
Those are calculated values (!), using as inputs the values in the second (DL,Mpc) and fourth (mbol) columns. They also validate ngc3314's conclusion ("a good match to the calculated 23.56-18.88=4.68 from the table, noting that the top point is not at z=0").

Now suppose our uniform 10 kpc disk galaxy emitted all its electromagnetic radiation as/at [OIII]5007. In a perfect astronomical observer's world, our galaxy would have a total, integrated magnitude of infinity (zero flux) in all but one of the five SDSS bands (and infinity in all, in the last two cases); the bands in which it would be visible would be, respectively, g, g, r, i, z, and z. In all bands except the z it would be detected, easily (though in the i band it'd be faint; it'd be too faint to be detected in the z band, even at z=0.75).

In a flat universe it would be an SDSS point souce in all cases except the first three, and in the third it'd be marginal (assuming a seeing of 1.4" in all bands); in the real universe, it would be a marginal point source in the fourth (z=0.5) case, and an SDSS point source at all greater z's.

Assuming a PSF of 0.05" for the HST, it would be easily resolved in all cases, in a flat universe and the real one. Whether it would be too faint to be detected I haven't checked, but I expect it to be easily detectable.

Now real galaxies emit light at many wavelengths, and that light is not emitted uniformly across a circular disk (the 'uniformly' is unreal, the 'circular disk' is not). The effect of the first is, obviously, to make our galaxy fainter than its apparent bolometric magnitude, and for it to have a non-infinite magnitude in all bands; the effect of the second is what the Disney and Lang paper is about.

Nereid
2012-Jan-20, 11:04 AM
A word about why I like ngc3314's approach.

His 'galaxy' is so horribly simplified it is almost a joke, wrt real disk (exponential) galaxies.

However, it's very easy - and intuitively straight-forward - to quickly turn it into a more reasonable toy (or cartoon) galaxy.

For example, add a bulge by adding a similar circular disk, with a radius of, say, 1 kpc.

Represent the colours of a real galaxy by splitting the -24 bolometric mags across five lines, centred in each of the five SDSS bands.

Represent an exponential disk by three co-centred disks of radius Re, 2 Re, and 3 Re (or some similar combo).

What say you, dear reader?

Nereid
2012-Jan-21, 02:26 PM
I made a mistake in my calculations in my last post. The SDSS seeing is 1.4" FWHM, and the -24 absolute (bolometric) mag disk has a radius of 10 kpc. Thus it remains distinguishable from a point source even at z=2 in the real universe; in a Euclidean universe (what I called "flat" in my last post), it becomes a point source at z=0.5.

One other thing to add: the venerable RC3 includes a field called "Log_D25", which has to do with the size of each galaxy in the catalogue as measured at the B-band 25th magnitude per arcsec squared isophote. In the toy universe I'll be exploring, astronomers can robustly detect and measure galaxy isophotes to 25 mag arcsec^-2, in all bands (and with the HST they can do a lot better). In the example in my last post, then, the galaxy would be observable, as an extended source, at all z's1.

Now consider the same, uniformly luminous, circular disk, but with an absolute bolometric magnitude of -17.7.

Here's the same table as before, with just the columns of interest:

z....mbol Bolometric SB
0.01 15.46 25.19
0.10 20.60 25.54
0.25 22.79 26.1
0.50 24.56 26.89
0.75 25.64 27.56
1.00 26.42 28.14
1.50 27.52 29.1
2.00 28.29 29.95


Now suppose this galaxy emits all its electromagnetic radiation at 265.0 nm. In the SDSS system, it is not visible at all until it has a z of 0.25 (among the eight cases above), and then only in the u-band. Its SB is too low for it to be detected, and even if it were a point source, it'd be too faint for the SDSS to have assigned it the honour of being a photometric object. At z=0.5 it would be detectable in both the u and g bands (if only it were brighter)! Why? Because all the SDSS bands overlap somewhat, and at 397.5 nm neither of the (u and g) filters transmits much. However, I'm assuming a perfect astronomical observer, so I'd have to decide to make it one band or the other ... If the galaxy were brighter it'd be visible in the g-band (z=0.75 and 1.00), r-band (1.5), and i-band (2).

1 Sorting out the apparent discrepancy of having a galaxy, as a point source, being too faint to be detected (e.g. SDSS g-band limit is 22.2 mag) , yet as an extended source being easily detectable will be left to another day.

Next, a somewhat brighter galaxy.

Jerry
2012-Jan-22, 01:30 AM
What say you, dear reader?

Let us start with the assumption we have a round cow...


(Enjoying the thread, and the thought that is going into it, thank you)

Nereid
2012-Jan-22, 01:45 AM
One more 'full treatment' galaxy: circular, uniformly luminous disk of radius 10 kpc; Mbol = -22.3

z....mbol Bolometric SB
0.01 10.86 20.59
0.10 16.00 20.94
0.25 18.19 21.50
0.50 19.96 22.29
0.75 21.04 22.96
1.00 21.82 23.54
1.50 22.92 24.50
2.00 23.69 25.30


Oh what a difference a mere 4.6 mags makes! This galaxy is easily detectable out to z=1, at 1.5 it would be too faint to detect if it were a point source in any SDSS band1 (but still marginal as an extended source), and invisible at z=2.

Now suppose this galaxy emits all its electromagnetic radiation at 666.5 nm. In the SDSS system, it is not visible beyond z=0.25 (among the eight cases above), and it moves through the bands quickly, going from r (at z=0.01) to i (at 0.10) to z (0.25).

You may be wondering why I've chosen these two toy galaxies; why, for example, pick Mbol = -17.7 and -22.3? Well, there is a method to my madness ... but first I've a few other galaxies to present for your pleasure.

Finally, a request: would some kind reader(s) please check my calculations (etc)?

1 Sorta; while the SB remains well above the threshhold, as a point source it'd be undetected by SDSS in both i and z bands at z=1 and z=1.5, and also at z=0.75 in the z-band. Of course, with an integration time per band of not even a minute (except for Stripe 82), and a wimpy 2.5m telescope, SDSS is far from being the deepest ground-based survey!

Nereid
2012-Jan-22, 07:06 AM
Thanks Jerry! :) The cows are actually spherical, but because we observe them from a great distance they appear circular.

Three more galaxies, but this time with data for only z=0.01, 0.25, 0.5, 1, and 2.

The first has an Mbol of -19.7:

z....mbol Bolometric SB
0.01 13.46 23.19
0.25 20.79 24.10
0.50 22.56 24.89
1.00 24.42 26.14
2.00 26.29 29.90


Hmm, is it a coincidence that these values are just the same as those for the Mbol of -17.7 galaxy, only 2 mags brighter? In any case, it would be detectable out to z~0.5.

Suppose this galaxy emits all its light at a wavelength of 350.0 nm; locally it would be visible in the SDSS u-band, at z=0.25 and 0.5 it'd be a g-band object, by z=1 an i-band one, at at z=2, invisible.

What if Mbol = -21.9?

z....mbol Bolometric SB
0.01 11.26 20.99
0.25 18.59 21.90
0.50 20.36 22.69
1.00 22.22 23.94
2.00 24.09 25.70


Much like the second galaxy; easily detectable out to z=1, invisible at z=2.

And if this galaxy shines only at 558.3nm? Then it starts as an r-band object, becomes an i-band one at z=0.25, a z-band one at 0.5, and is invisible thereafter.

Finally, for now, Mbol = -21.3. As this is just 1 mag fainter than the second galaxy (Mbol = -22.3), I'll leave you, dear reader, to work out how far out it has to be before it 'has completely sunk' (to use D&L's term). Let's see what happens if this galaxy emits all its light at 460.0 nm: locally it's a g-band object, at z=0.25 an r-band one, at 0.5 an i, at z=1 a z-band object, and invisible at z=2.

Next: what if all five galaxies were, in fact, just the same galaxy?

Nereid
2012-Jan-23, 12:46 PM
[...] But for extended sources, it's more usual (and germane to galaxy detection) to deal with surface brightness, at some reference radius or within some metric radius. [...] consider an object with bolometric magnitude Mbol=-24. The apparent bolometric magnitude mbol is just -24 + 5 log (D/10 pc). To show surface brightness, let this flux be uniformly distributed in a circular disk with radius 10 kpc (thus over an area pi*(10/angular scale)^2 arcsec^2). [...]
To get a feel for just how faint galaxies are, imagine that the centre of this Mbol=-24, radius 10 kpc disk is 10 pc away (and perpendicular to a line from us to that centre). It would fill half the sky!

But could you see it, with your own eyes, on a clear, moonless night? After all, its surface brightness is only 18.81 mag/arcsec^2, despite the fact that, at 10 pc from us, its total light would be a mere 2 magnitudes (or so) fainter than the Sun.

Obviously, only if it emitted light in that part of the electromagnetic spectrum your eyes are sensitive to (that would rule out the first of my galaxies, even if it weren't so faint; none of its light - which is in the UV - would even get to the ground). Suppose all the light was emitted close to the part of spectrum which our eyes are most sensitive to. For dim light, our eyes' PSF is approx 4' (radius), say. In this case, the centre of this magnificent disk galaxy, a mere 10 pc away, would be visible with the unaided eye ... just. I get an integrated magnitude of ~5.7! (please check my calculations).

ngc3314
2012-Jan-23, 01:38 PM
A quick way to check the scaling: the full Moon, at apparent visual magnitude -12.7 and angular area around pi/16~0.20 square degrees, has surface brightness of V=-12.7+2.5 log (0.20) = -14.44. Or in the usual galaxy units of equivalent magnitude, we have to add 5 log (3600) = 17.78 to get V=3.3 per square arcsecond, which makes sense from comparing visibility of planets and bright stars near occultation. So our toy galaxy is 15.5 magnitudes fainter than that, which would, for example, correspond to a source with the Moon's angular size but visual magnitude 2.8. That's not too different from the Small Magellanic Cloud (NED lists integrated V=1.87 over an area 185x320 arcminutes ~ 12.9 square degrees, which would mean V=22.4 per square arcsecond (of course, the inner parts which are visually obvious are rather brighter than this). For such extended sources, one can do much better than the otherwise equivalent point-source limit (visually and instrumentally) because detection averages over a much larger region than the PSF.

Nereid
2012-Jan-23, 02:25 PM
A quick way to check the scaling: the full Moon, at apparent visual magnitude -12.7 and angular area around pi/16~0.20 square degrees, has surface brightness of V=-12.7+2.5 log (0.20) = -14.44. Or in the usual galaxy units of equivalent magnitude, we have to add 5 log (3600) = 17.78 to get V=3.3 per square arcsecond, which makes sense from comparing visibility of planets and bright stars near occultation. So our toy galaxy is 15.5 magnitudes fainter than that, which would, for example, correspond to a source with the Moon's angular size but visual magnitude 2.8. That's not too different from the Small Magellanic Cloud (NED lists integrated V=1.87 over an area 185x320 arcminutes ~ 12.9 square degrees, which would mean V=22.4 per square arcsecond (of course, the inner parts which are visually obvious are rather brighter than this).
Thanks!

Using an apparent visual magnitude (i.e. V-band) of -12.7 for the Moon, and a half degree diameter, I get V=3.3 per arcsecond squared (to one decimal place). A patch of sky the size of the Moon, at the centre of our toy galaxy, 10 pc away, assuming it emits all its light in the V-band, would have an apparent V-band magnitude of 2.8.


For such extended sources, one can do much better than the otherwise equivalent point-source limit (visually and instrumentally) because detection averages over a much larger region than the PSF.

For the purposes of this thread, just how much better, and why, are important.

Earlier, I wrote:

Sorting out the apparent discrepancy of having a galaxy, as a point source, being too faint to be detected (e.g. SDSS g-band limit is 22.2 mag) , yet as an extended source being easily detectable will be left to another day.
A circular area, radius 2.1", of a patch of sky with a uniform surface brightness of 25 mag/arcsec^2, has an integrated magnitude of 22.2.

First, though, let's explore (observe) my toy galaxies galaxy a bit more.

Put all five of the galaxies together. They would have an Mbol of -23.1, and the bolometric SB locally would be 19.7 mag/"^2.

StupendousMan
2012-Jan-23, 03:06 PM
So, how easy is it to detect a faint object against the sky?

If the object's surface brightness is larger than that of the sky, then it's easy: it sticks out in an obvious way. Done. Example: a perfectly still lake has a surface which is perfectly flat. A turtle sticks its head up so that it is 1 inch above the surface. "Look -- there it is!"

If the object's surface brightness is less than that of the sky, it's still possible. The key is whether the surface brightness of the object is larger than fluctuations in the sky background. If the answer is "yes", then it's still possible without great effort. Just model the background, subtract it, and look for items which exceed the level of the residuals. Example: a lake has waves which are amazingly perfect sinusoids -- there are no ripples or foam, just a smooth surface which rises and falls in a perfectly regular way with an amplitude of 1 foot. A turtle sticks its head up so that it is 1 inch above the surface. That does not stand out at first glance, because the waves are much larger. But, if one takes a picture, makes a model of the waves, and subtracts the model -- the leftovers will be a perfectly flat surface. The turtle's head is the only positive residual from the model. "There it is!"

If the object's surface brightness is smaller than fluctuations in the background, then detecting it is hard. Example: a lake has realistic waves, which are basically sinusoidal, but with small ripples and foam irregularities of size around 2-3 inches. A turtle sticks its head 1 inch above the water. You take a picture, make the best model you can, subtract it from the picture ... and the leftover water surface still has ripples and holes which are larger than 1 inch in size. "Where is that turtle?"

Nereid
2012-Jan-24, 11:55 AM
Am limited today, and tomorrow, so just a couple of brief words.

Thanks SM, and ngc3314,;this aspect (instrumental detectability of low SB objects) is something which I will certainly be interested in exploring more deeply, but later.

Any word from the others who've posted in this thread previously? Recently Jerry, ngc3314, StupendousMan, parejkoj, and antoniseb have posted; what about you others?

Specifically, George: what do you think about the colours of the toy galaxy?

Suppose T1 - toy galaxy #1, introduced by ngc3314, with an Mbol=-24, uniformly luminous circular disk, radius 10 kpc - were the surface of a sphere, at a distance of 10 pc from us, emitting all its electromagnetic radiation in the V-band. Would the dark, cloudless/clear, moonless night sky be sufficed with a uniform glow, of apparent magnitude ~2.8?

I'm not sure how much cloud/fog/smog would be needed, or how early in the morning/late in the afternoon (etc), but the Sun's apparent visual magnitude is surely often very close to -24; spreading this much light over the whole sky (both day and night) would - if I understand it correctly - merely make the sky glow like the diffuse light from a nearby large city (in my personal experience, it doesn't take much to drown out the SMC, and M31 is even easier to sink).

ngc3314
2012-Jan-24, 10:35 PM
Suppose T1 - toy galaxy #1, introduced by ngc3314, with an Mbol=-24, uniformly luminous circular disk, radius 10 kpc - were the surface of a sphere, at a distance of 10 pc from us, emitting all its electromagnetic radiation in the V-band. Would the dark, cloudless/clear, moonless night sky be sufficed with a uniform glow, of apparent magnitude ~2.8?

I'm not sure how much cloud/fog/smog would be needed, or how early in the morning/late in the afternoon (etc), but the Sun's apparent visual magnitude is surely often very close to -24; spreading this much light over the whole sky (both day and night) would - if I understand it correctly - merely make the sky glow like the diffuse light from a nearby large city (in my personal experience, it doesn't take much to drown out the SMC, and M31 is even easier to sink).

One reference point - the typical light of the daytime sky is the redistributed version of (most of the) >25% or so of the light of the Sun which is extinguished[1] by the atmosphere at a typical sea-level site. So an apparent magnitude -24 spread around the sky would be pretty bright, much brighter than the scattered light of the night sky at full Moon. This does get unrealistic - we shod treat the surface brightness itself, since our distance changes so much from different regions of that imaginary disk centered 10 pc away that the mapping gets complicated - the mean distance from each piece of it is pretty large. Surface brightness is the same at mag=18.81 per square arcsecond, but total flux gets strange. Working back from that we get an equivalent magnitude all over that hemisphere of 18.81-10 log (60)-2.5 log (20600) = -9.7. If in the V band, that would be very roughly the brightness of a typical sky when the Moon is up just slightly after first quarter (using the 25% rule from sunlight).

[1]Weasel words because extinction includes actual absorption and scattering, and the mix changes with wavelength as well as overall scaling with secant[2] of the angular distance from zenith.

[2] Slight swindle, secant assumes the Earth and atmosphere are flat.

Nereid
2012-Jan-30, 09:16 PM
Suppose T1 - toy galaxy #1, introduced by ngc3314, with an Mbol=-24, uniformly luminous circular disk, radius 10 kpc - were the surface of a sphere, at a distance of 10 pc from us, emitting all its electromagnetic radiation in the V-band. Would the dark, cloudless/clear, moonless night sky be sufficed with a uniform glow, of apparent magnitude ~2.8?

I'm not sure how much cloud/fog/smog would be needed, or how early in the morning/late in the afternoon (etc), but the Sun's apparent visual magnitude is surely often very close to -24; spreading this much light over the whole sky (both day and night) would - if I understand it correctly - merely make the sky glow like the diffuse light from a nearby large city (in my personal experience, it doesn't take much to drown out the SMC, and M31 is even easier to sink).One reference point - the typical light of the daytime sky is the redistributed version of (most of the) >25% or so of the light of the Sun which is extinguished[1] by the atmosphere at a typical sea-level site. So an apparent magnitude -24 spread around the sky would be pretty bright, much brighter than the scattered light of the night sky at full Moon. This does get unrealistic - we shod treat the surface brightness itself, since our distance changes so much from different regions of that imaginary disk centered 10 pc away that the mapping gets complicated - the mean distance from each piece of it is pretty large. Surface brightness is the same at mag=18.81 per square arcsecond, but total flux gets strange. Working back from that we get an equivalent magnitude all over that hemisphere of 18.81-10 log (60)-2.5 log (20600) = -9.7. If in the V band, that would be very roughly the brightness of a typical sky when the Moon is up just slightly after first quarter (using the 25% rule from sunlight).

[1]Weasel words because extinction includes actual absorption and scattering, and the mix changes with wavelength as well as overall scaling with secant[2] of the angular distance from zenith.

[2] Slight swindle, secant assumes the Earth and atmosphere are flat.
These last few weeks my internet access has been, um, unpredictable. I had taken to preparing all my responses offline ... but in this case I made an exception.

Of course, the moment I pressed "Post", I realised I had made several, pretty enormous, mistakes! :doh: :o

The toy galaxy, T1, is huge! It has a radius of 10 kpc!!

My musing - in the post quoted by ngc3314 - concerns a completely different toy galaxy, an almost laughably small one: although they both have the same bolometric absolute magnitude (absolute bolometric magnitude?), -24, their surface areas are wildly different. T1's is ~3x10^8 square parsecs, my 'musing' one, ~1.3x10^3 pc^2.

Assuming no absorption between us and the (puny) toy galaxy - and that includes the Earth's atmosphere - the surface brightness would be ~5.3 mag per "^2; a 'Moon's worth' - directly overhead - would shine at an integrated magnitude of ~-10.7.

Anyway, back to doing some real astrophysics, albeit with toy galaxies.

Nereid
2012-Feb-01, 05:23 PM
To keep track of the various toy galaxies, I've started numbering/naming them: Tn (for Toy galaxy, number)

So far there are nine so far (click on the name to get the post in which it was first introduced, in this thread):

Tn..Mbol...r1...SB2...SED3; note
-- ----- ---- ----- ---------
T1 (http://www.bautforum.com/showthread.php/120972-Bias-effects-in-galaxy-detection?p=1980325#post1980325) -24.0 10.0 18.81 unspecified
T2 (http://www.bautforum.com/showthread.php/120972-Bias-effects-in-galaxy-detection?p=1981143#post1981143) -24.0 10.0 18.81 [OIII] 5007
T3 (http://www.bautforum.com/showthread.php/120972-Bias-effects-in-galaxy-detection?p=1981969#post1981969) -17.7 10.0 25.12 265.0 nm
T4 (http://www.bautforum.com/showthread.php/120972-Bias-effects-in-galaxy-detection?p=1982176#post1982176) -22.3 10.0 20.52 666.5 nm
T5 (http://www.bautforum.com/showthread.php/120972-Bias-effects-in-galaxy-detection?p=1982239#post1982239) -19.7 10.0 23.12 350.0 nm
T6 (http://www.bautforum.com/showthread.php/120972-Bias-effects-in-galaxy-detection?p=1982239#post1982239) -21.9 10.0 20.92 558.3 nm
T7 (http://www.bautforum.com/showthread.php/120972-Bias-effects-in-galaxy-detection?p=1982239#post1982239) -21.3 10.0 21.52 460.0 nm
T8 (http://www.bautforum.com/showthread.php/120972-Bias-effects-in-galaxy-detection?p=1982587#post1982587) -23.1 10.0 19.72 265, 350, 460, 558.3, 666.5 nm; T3 to T7 combined
T9 (http://www.bautforum.com/showthread.php/120972-Bias-effects-in-galaxy-detection?p=1982908#post1982908) -24.0 0.01 05.32 V-band
-- ----- ---- ----- ---------


1 all the galaxies are uniformly luminous, circular disks (unless otherwise noted); radius (in kpc)
2 local, mag per arcsec^2
3 Spectral Energy Density (or Distribution), in shorthand (refer to the originating post for details)

Nereid
2012-Feb-01, 06:15 PM
First, though, let's explore (observe) my toy galaxies galaxy a bit more.

Put all five of the galaxies together. They would have an Mbol of -23.1, and the bolometric SB locally would be 19.7 mag/"^2.

This is T8.

Locally its colours would be, in an idealised, perfect SDSS system:

u-g: 1.6
g-r: 1.6 (T8's absolute r-band magnitude is -22.9 (-22.86 actually), can you see why?)
r-i {meaningless; T8 does not emit in the i-band}
i-z {meaningless; T8 does not emit in the i-band, nor the z-band}

So this galaxy would appear rather red (though perhaps not to the unaided eye of course, even when viewed through the eyepiece of a really big telescope; do you know why?)

If we were to try to approximate this toy galaxy's SED - as measured by those two colours alone - as a blackbody, what temperature would it have? What if we added the NUV emission, at 265.0 nm?

If observed at a distance of z=0.25, its apparent colours would be:
u-g: 2.0
g-r: 1.6
r-i: 0.6
i-z: 0.4

These would shift red-ward, by one colour, at z=0.75.

And blue-ward, also by one colour, at z=0.1.

At other redshifts in our table (0.5, 1, 1.5, and 2) the colours would be rather unnatural; e.g. no flux in the r-band at z=0.5 and z=1 (at z=2, the only band with flux would be the z-band).

Could someone please check my calculations?

Nereid
2012-Feb-02, 11:14 PM
I hope, by now, you - dear reader - will see that Disney&Lang's conclusions are based on evidence that even my (and ngc3314's) toy galaxies shows is silly ridiculous not of this universe.

Take, as a very simple example, the fact that the redshift at which even the SDSS u-band disappears from that wildly successful five-band survey is relatively modest, cosmologically speaking ... and that it's even more dramatic if you restrict yourself - as Disney seems to imply - to the workhorse of astronomical surveys of the 1960s (or thereabouts); namely, photographic plates and Schmidt telescopes.

Let's explore observability, sinking, etc with the HST. What, for example, are the perfect (toy universe) pass-bands routinely used by ACS? What is the sky (in mags per arcsec squared), in those pass/wave-bands? How deep can the HST+ACS go, in survey mode?

George
2012-Feb-03, 04:07 AM
This is T8.

Locally its colours would be, in an idealised, perfect SDSS system:

u-g: 1.6
g-r: 1.6 (T8's absolute r-band magnitude is -22.9 (-22.86 actually), can you see why?)
r-i {meaningless; T8 does not emit in the i-band}
i-z {meaningless; T8 does not emit in the i-band, nor the z-band}

So this galaxy would appear rather red (though perhaps not to the unaided eye of course, even when viewed through the eyepiece of a really big telescope; do you know why?)[Assuming I understand that the emissions are monochromatic...]

It seems white to me since you have red (T4), green (T6) and blue (T7) emissions, but the blue and green are stronger, similar to sunlight, which is white.



If observed at a distance of z=0.25, its apparent colours would be:
u-g: 2.0
g-r: 1.6
r-i: 0.6
i-z: 0.4
I think this changes it to violet, red and greenish-yellow. I don't know off-hand what color this result would be.


At z = .5, you have only green and red, which might produce a yellowish color.

At z = .75, you have blue and orange, which should give some sort of greenish color I suspect.

At z = 1, you have green and red, which might give you a yellowish or orangish appearance.



At other redshifts in our table (0.5, 1, 1.5, and 2) the colours would be rather unnatural; e.g. no flux in the r-band at z=0.5 and z=1 (at z=2, the only band with flux would be the z-band). Wouldn't T3 be red from z=1.5 to almost 2?

antoniseb
2012-Feb-03, 02:27 PM
... Wouldn't T3 be red from z=1.5 to almost 2?
There are UV photons being redshifted into the visible spectrum as well. The whole Lyman series most prominently, but other transitions as well. Once you get beyond Z=1, the Lyman series starts showing up as blue-indigo.

ngc3314
2012-Feb-03, 03:36 PM
Once you get beyond Z=1, the Lyman series starts showing up as blue-indigo.

Well, z>1.6 to get Lyman alpha past the atmospheric cutoff around 3200 A. (Picky, picky).

Continuum starlight, rather than emission lines, is the only important thing in normal galaxies in the emitted UV - there aren't any more strong emission lines until you get to Lyman alpha (which is grotesquely sensitive to internal radiative-transfer effects) for non-AGN.

George
2012-Feb-03, 03:47 PM
There are UV photons being redshifted into the visible spectrum as well. The whole Lyman series most prominently, but other transitions as well. Once you get beyond Z=1, the Lyman series starts showing up as blue-indigo.I may be in the weeds relative to the path of this thread since I haven't had time to absorb the interesting and educational content this thread provides. As usual, however, I will exercise my amateur privledge and throw out what little I have in my back pack...

If we have a 265 nm emission (T3), then a redshift of 1.5 will produce a red color observation (662 nm). At z = 2, then the observed wavelength becomes 795 nm, which is just past the observable red for many, though some can see this wavelength, reportedly. [I used the simple non-relativistic formula of z+1 = lambdaobs / lambdaemit.]

George
2012-Feb-03, 03:56 PM
Well, z>1.6 to get Lyman alpha past the atmospheric cutoff around 3200 A. (Picky, picky).

Continuum starlight, rather than emission lines, is the only important thing in normal galaxies in the emitted UV - there aren't any more strong emission lines until you get to Lyman alpha (which is grotesquely sensitive to internal radiative-transfer effects) for non-AGN. Oops, I missed this post. So are the stated wavelengths for the "toys" the emission lines and not an imaginary bulk emission at those wavelengths? [I just knew I had to be in the weeds, or, at best, forbes.]

Nereid
2012-Feb-03, 04:33 PM
Oops, I missed this post. So are the stated wavelengths for the "toys" the emission lines and not an imaginary bulk emission at those wavelengths? [I just knew I had to be in the weeds, or, at best, forbes.]

The toy emission lines are unreal (or, to be a bit more picky, unreal for real galaxies; I'm sure there are atomic transitions at, or near, my toy lines, but no real galaxy emits these, as dominating its SED, even within a band1).

I intend to introduce a somewhat more satisfactory set of toy lines later, so that we will not ever encounter the rather silly "Locally its colours would be, in an idealised, perfect SDSS system: [...] r-i {meaningless; T8 does not emit in the i-band}; i-z {meaningless; T8 does not emit in the i-band, nor the z-band}".

To recap: the toy astronomy I'm doing so far is only imaging and photometry, and most of it is done using an idealised, perfect SDSS system of pass/wavebands (I hope to introduce something similar, from the Hubble's ACS; maybe later this week?). This is consistent with the Disney and Lang paper ngc3314 introduced, in the OP: that paper is totally blind to spectroscopy (though spectrophotometry is marginally relevant, if only implicitly).

1 There's some interesting stuff here, that I hope to pursue later, concerning real galaxies; hint: why does Hanny's Voorwerp appear pure blue in the images from SDSS' CAS (http://skyservice.pha.jhu.edu/DR7/ImgCutout/getjpeg.aspx?ra=145.26667&dec=34.73278&scale=0.19806&opt=&width=512&height=512)? If you could see it with your own eyes, what colour would it appear?

Nereid
2012-Feb-03, 05:18 PM
[Assuming I understand that the emissions are monochromatic...]

It seems white to me since you have red (T4), green (T6) and blue (T7) emissions, but the blue and green are stronger, similar to sunlight, which is white.

I think this changes it to violet, red and greenish-yellow. I don't know off-hand what color this result would be.


At z = .5, you have only green and red, which might produce a yellowish color.

At z = .75, you have blue and orange, which should give some sort of greenish color I suspect.

At z = 1, you have green and red, which might give you a yellowish or orangish appearance.


Wouldn't T3 be red from z=1.5 to almost 2?

Let's explore this a bit more ...

The central wavelengths of the SDSS bands (http://www.astro.princeton.edu/PBOOK/camera/camera.htm) are, approximately:
u 354 nm
g 477 nm
r 623 nm
i 763 nm
z 913 nm

A sufficiently bright light, at each of those wavelengths, would be perceived, by a person with normal vision, as ...?

The SDSS bands overlap somewhat; the transitions occur at approximately:
u/g 395 nm
g/r 550 nm
r/i 689 nm
(no need for the i/z one; humans' eyes are blind to that!)

A sufficiently bright light, at each of those wavelengths, would be perceived, by a person with normal vision, as ...? (This will give us an idea of how perceived colour varies, across each SDSS band).

If you look at a colour (u-g) - colour (g-r) plot, of stars, or galaxies, or quasars, are there many (any?) near (1.6, 1.6)? If so, finding one of those objects will tell us what colour T8 would appear, locally.

At z = 0.25, the region of this colour-colour plot we're interested in is around (2.0, 1.6).

George
2012-Feb-03, 08:19 PM
The toy emission lines are unreal (or, to be a bit more picky, unreal for real galaxies; I'm sure there are atomic transitions at, or near, my toy lines, but no real galaxy emits these, as dominating its SED, even within a band1). [my bold] Somehow I missed this description. I prefer laser [monochromatic] toys, especially the size of galaxies. :)

Visual color, of course, is never determined by emission lines, so I would assume. Color is simply the result of the integration of the visible SED, though more appropriately represented by a photon flux distribution, IMO, since blue peaks in a SED can mislead one into thinking a blue color might be possible.


To recap: the toy astronomy I'm doing so far is only imaging and photometry, and most of it is done using an idealised, perfect SDSS system of pass/wavebands (I hope to introduce something similar, from the Hubble's ACS; maybe later this week?). This is consistent with the Disney and Lang paper ngc3314 introduced, in the OP: that paper is totally blind to spectroscopy (though spectrophotometry is marginally relevant, if only implicitly). As I feared, I need to get my head around the whole subject matter before I advance my foot-in-mouth disease... Or not...

If spectrometry is no longer a problem, shouldn't the SED be the ideal story teller of any object? Obviously, the filter sets are wonderful if no spectrometry is obtainable, or perhaps, their are greater problems for spectrometry with extended objects. I'm just guessing that SEDs should be the more modern view due to advanced technologies, especially given the Sloan work. [Perhaps they are.]


1 There's some interesting stuff here, that I hope to pursue later, concerning real galaxies; hint: why does Hanny's Voorwerp appear pure blue in the images from SDSS' CAS (http://skyservice.pha.jhu.edu/DR7/ImgCutout/getjpeg.aspx?ra=145.26667&dec=34.73278&scale=0.19806&opt=&width=512&height=512)? If you could see it with your own eyes, what colour would it appear? Well, you couldn't have picked a more appropriate, though rarther unique, example, huh ngc3314? :)

Very few extended objects in the universe exhibit color. The eye's sensitivity range of about 12 orders does not apply to color sensitivity, so we lose the ability to see color for the dimmer objects, especially nebulae. There are exceptions, of course. I have seen the blue ring of the "bright" Eskimo nebula using an 82" telescope.

The other problem is the fact that if we magnify or even travel closer to these objects, we essentially gain no improvement in being able to see color. Surface brightness and size both obey the inverse square law. Traveling half the distance toward the object yields 4x the amount of light but it will appear 4x as large, so if it was grey when you left, it will still look grey. [It took Ken G an embarassing no. of posts to convince me no optical device could alter this circumstance to the point we bet an ice cream Sundae. A freakish technicallity emerged and, well, I owe him a steak, but owes me an ice cream Sundae. :)]

[Hopefully I'm on topic.]

George
2012-Feb-03, 09:08 PM
1 There's some interesting stuff here, that I hope to pursue later, concerning real galaxies; hint: why does Hanny's Voorwerp appear pure blue in the images from SDSS' CAS (http://skyservice.pha.jhu.edu/DR7/ImgCutout/getjpeg.aspx?ra=145.26667&dec=34.73278&scale=0.19806&opt=&width=512&height=512)? If you could see it with your own eyes, what colour would it appear? I'm curious also about Hanny's Voorwerp possible visual color. It does seem bright.

I think I'm correct in saying that if any region of a nebula has a local surface brightness brighter than around 5 mag. per sq. arcminute (~ 14 mag. per sq. arcsec) then color is possible, and if there is a significant bulge in the visual band of the SED that would allow a specific color to emerge. The Eskimo, for instance, has a surface brightness average of about 6.8 mag (sq. arc. min.)

ngc3314
2012-Feb-03, 09:17 PM
It does seem bright.

Not so much! I'm aware of a couple of people who have seen it visually, all using Jimi Lowrey's personal 1.2m Dob in West Texas. Jimi did detailed enough sketches to tell that unfiltered he was mostly seeing the continuum star-forming regions, while an SDSS g filter brought out the gas. I can't picture a normal human detecting the color (which would, based on the spectrum dominated by redshifted [O III], be distinctly greenish).

Side note: my eyes seem to see [O III] somewhat bluer with age - 30-odd years ago, NGC 7027 was almost emerald green, while more recent observations of planetaries with a friend's 62-cm Dob give them more of a bluish-green tint.

George
2012-Feb-03, 09:42 PM
A sufficiently bright light, at each of those wavelengths, would be perceived, by a person with normal vision, as ...? (This will give us an idea of how perceived colour varies, across each SDSS band).
u 354 nm UV, not visible
477 nm Light blue, or more blue saturated cyan
623 nm Orangish-red
763 nmRed, but many likely can not quite see it, though some can.
913 nm NIR, not visible

The color spectrum chart in Wiki (http://en.wikipedia.org/wiki/Visible_spectrum) (about half way down the page) is about as accurate as I think it should be. When I first wondered what wavelengths produced each color sensation, I discovered a surprising variation in the color charts. The heliochromolgist took an average of many charts to help establish a likely spectral coloring chart. Wiki's chart seems to match fairly close to that of the heliochromologist, so it's probably wrong. ;)

[Perhaps there are a few major research papers on this. ]


The SDSS bands overlap somewhat; the transitions occur at approximately:
u/g 395 nm
g/r 550 nm Unfortunately, this is essentially half of the visual color spectrum, from violet to green. (Green lasers are 532nm)

g/r 550 nm
r/i 689 nm And this is the other half. [Yellow and orange are very narrow bands within the spectrum]

Leave it to astronomers to focus on scientific efficacy, and not side asterochromolgical issues! *wink*


A sufficiently bright light, at each of those wavelengths, would be perceived, by a person with normal vision, as ...? (This will give us an idea of how perceived colour varies, across each SDSS band). The broad band of these filters, in spite of my spite, should reveal the two typical celestial colors: bluish-white and orange (white doesn't count). I suspect there are few yellow stars out there. The hot ones are bluish-white (never a saturated blue) and the cool ones are orange. [Some (e.g. Grant) see Antares as red, but I see it as orange. What do you see?]

Planck distributions for stars reveal why most are white stars since no one color dominates all that much. Green is all but impossible, though yellow is since green and orangish-red augment the tiny yellow band.

George
2012-Feb-03, 09:57 PM
Not so much! I'm aware of a couple of people who have seen it visually, all using Jimi Lowrey's personal 1.2m Dob in West Texas. Jimi did detailed enough sketches to tell that unfiltered he was mostly seeing the continuum star-forming regions, while an SDSS g filter brought out the gas. I can't picture a normal human detecting the color (which would, based on the spectrum dominated by redshifted [O III], be distinctly greenish). Well the images y'all produced sure make it look cool regardless! I do not know of many objects that do reveal color, though there are some including a few you have mentioned in the past. 14 mag. per sq. arc sec. is rare for any region of any extended object, no doubt.


Side note: my eyes seem to see [O III] somewhat bluer with age - 30-odd years ago, NGC 7027 was almost emerald green, while more recent observations of planetaries with a friend's 62-cm Dob give them more of a bluish-green tint. 500 nm is almost cyan. Feel better? Also, tornadic dust will favor greater blue scattering, or perhaps residual hot air after a recent football game [or both] are contributing factors. :)

Nereid
2012-Feb-03, 10:22 PM
To make the toy astronomy more consistent, somewhat more real, and easier to work in, I've re-worked my workhorse (a spreadsheet), and re-designed my (toy) filter set.

In this post, the toy filter set.

There are ten filters in all, and I've given them the following names (in order of increasing wavelength): n1t, n2t, ut, gt, rt, it, zt, IR1t, IR2t, and IR3t (If anyone would like to suggest a better set of names, I'd be happy to adopt it!).

The cutoffs between toy bands are perfectly sharp (no overlaps, unlike the real SDSS ones, for example), and there are no gaps. The long wavelength end of each of the ten bands is as follows (in nm): 192.0, 327.0, 405.5, 533.0, 675.5, 821.8, 985.5, 1185.5, 1452.1, and 1777.8. The lower (short) wavelength cutoff for the n1t band is 114.0 nm.

I'll also be extending the redshift coverage of my toy astronomy, to z=6. Here are the extra entries in the table (following ngc3314 (http://www.bautforum.com/showthread.php/120972-Bias-effects-in-galaxy-detection?p=1980325#post1980325)); i.e. T1.

z...DL,Mpc ang scale, kpc/"" mbol Bolometric SB
3.0 25842 . . 7.83 . . . . . 23.06 24.83
4.0 36524 . . 7.08 . . . . . 23.81 25.80
5.0 47594 . . 6.41 . . . . . 24.38 26.59
6.0 58951 . . 5.83 . . . . . 24.85 27.26

Next: revised 'visibility' of T8 (which, automatically, also includes T3 to T7).

Nereid
2012-Feb-04, 12:33 AM
Thanks very much for you comments, George! :)

Rather than try to comment on my rather muddled earlier colours, I'll use "[t]he color spectrum chart in Wiki (about half way down the page) is about as accurate as I think it should be" to describe the colour ranges of my new toy filters/bands.

As expected, the first two - n1t and n2t - and the last four - zt, IR1t, IR2t, and IR3t - are totally outside the range of human colour vision/perception.

The upper (long) wavelength of the ut band (405.5 nm) falls in the Wiki page's violet (can that colour be reproduced, in anything approaching faithfulness, in this forum?)

The gt band stretches from violet to green (405.5 to 533.0 nm); perhaps we could represent this, as a default, as cyan?

And rt from green to red (533.0 to 675.5 nm); would orange be OK as a default, for this band?

The it band is entirely in the red.

Antares? The last time I really paid attention (rather too many years' ago, I'm sorry to say), it seemed reddish-orange. I do recall - vaguely - noticing that it seemed redder when low down (close to the horizon).

Galaxy colours - if only you could see them (they're generally far too faint, and the SB far too low)! - can be quite different from stars'. For example, there can be light from galaxies almost completely dominated by emission lines. As redshift increases, these move red-ward (and new lines come into view). There are some interesting threads in the Galaxy Zoo forum on this (though they are more about the appearance in the CAS gri->BGR mapped false colours than what you'd see if only your eyes were sensitive enough). Zooites - as they call themselves - found some quite remarkable objects.

Nereid
2012-Feb-04, 01:25 AM
As promised, the 'visibility' of the toy galaxy, T8, in the revised toy filter/bands, at various redshifts.

Recall that T8 has an Mbol of -23.1, and emits all its light in five lines - at restframe wavelengths of 265, 350, 460, 558.3, and 666.5 nm.

Locally, and at z=0.01, T8 would be visible in just four of the toy bands: nt2, ut, gt, and rt (visible in the sense that light emitted by it could, potentially, be detected; whether it would actually be detected depends - obviously - on details of the toy telescope+photometer we use to observe it). Note that the emission at 558.3 and 666.5 nm would both be detected in the rt band.

At z = 0.1, we have five-band visibility: nt2, ut, gt, rt, and it.

Ditto at z = 0.25, also five-band visibility, in a game of 'musical bands': ut, gt, rt, it, and zt.

At z = 0.5, ut and gt remain, but 460 nm moves into the it band, and the other two lines also play musical bands; i.e. they move into the zt and IR1t bands.

Back to five contiguous bands at z = 0.75: gt, rt, it, zt, and IR1t.

A 'drop-out' band re-appears at z = 1: the 350 nm line is in the it band, while the 265 nm one remains in the gt band; otherwise, musical bands again: gt, it, zt, IR1t, IR2t.

By z = 1.5, we see the last appearance of the 666.5 nm line, in the IR3t band, and another drop-out, this time in the it band. The line-up is rt, zt, IR1t, IR2t, and IR3t.

At z = 2, the last of ngc3314's redshifts, the drop-out is in the zt band, and we've lost the 666.5 line: it, IR1t, IR2t, and IR3t.

Only two lines/bands remain at z = 3, 265 nm visible in the IR1t band, and 350 nm in the IR2t band. These move to the IR2t and IR3t bands, respectively, at z = 4.

Finally, at z = 5, just one colour/band remains, IR3t.

At z = 6 T8 has vanished into the infrared, beyond the capabilities of our toy, ten-band, astronomy.

Next: when does T8 become too faint to be detected, in any band? When does its surface brightness drop below 25 (mag/arcsec^2)?

Oh, and does the fact that no one has commented mean that you've all checked my calculations, and found no errors? ;)

George
2012-Feb-04, 01:29 AM
The upper (long) wavelength of the ut band (405.5 nm) falls in the Wiki page's violet (can that colour be reproduced, in anything approaching faithfulness, in this forum?)
The color html code #AA3BFF should come fairly close. It just so happens that 405 nm is another laser wavelength.

[quote]The gt band stretches from violet to green (405.5 to 533.0 nm); perhaps we could represent this, as a default, as cyan? I fear I'm missing where this is suppose to be going. [I've been a lousy lurker, admittedly.] What is it you (ya'll) are trying to accomplish? Doesn't the SED say it all now that SDSS data is available?


Antares? The last time I really paid attention (rather too many years' ago, I'm sorry to say), it seemed reddish-orange. I do recall - vaguely - noticing that it seemed redder when low down (close to the horizon).
Of course, the extensive scattering effects near the horizon will make most things more red. [It is rare to see a very red Sun, surprisingly, especially at sunrise.]

Here is an interesting image of Antares...
16263

... just before it entered our atmosphere and exploded, apparently. :rolleyes:

[How do we reduce the image size??]

I think I was trying a defocusing technique to avoid over exposure in order to capture its "true" color. It does happen to be the color I see it when defocused through the scope, and at a high altitude.

Again, because stars are close to bb radiators, even strong red emissions get blueshifted into orange because of the semi-strong emissions in the rest of the visible spectrum. T-class stars are certainly an exception due to molecular color "magic".


Galaxy colours - if only you could see them (they're generally far too faint, and the SB far too low)! - can be quite different from stars'. Yes, but what about the "hot" spots shortly after a million stars are borne from a GMC? And what about the nuclei of quasars or AGN? If I'm right (and I'm never always wrong), all you need is a region > a couple of sq. arcminutes that exceeds about 14 mag. per sq. arcminute to potentially present a color result.


For example, there can be light from galaxies almost completely dominated by emission lines. I didn't know this. This would mean, I assume, that the shroud of gas around these galaxies has all but elimiated the other portions of the spectrum? If so, they must be quite dim, so even if it was OIII, it would be too dim to be seen and appreciated as green. I hear it is tough being green. ;)


As redshift increases, these move red-ward (and new lines come into view). There are some interesting threads in the Galaxy Zoo forum on this (though they are more about the appearance in the CAS gri->BGR mapped false colours than what you'd see if only your eyes were sensitive enough). Zooites - as they call themselves - found some quite remarkable objects. False coloring is almost always better. I saw a true color for the "Pillars of Creation" and the result is rather a sickly red and orange columnated blobs. Hester et. al. false coloring allows for excellent revelations of the distinctive primary gases, with the side benefit of looking great.

Nereid
2012-Feb-04, 02:42 AM
I fear I'm missing where this is suppose to be going. [I've been a lousy lurker, admittedly.] What is it you (ya'll) are trying to accomplish? Doesn't the SED say it all now that SDSS data is available?

I can't say what others who are active here are trying to accomplish; however, I would like to demolish as many of the key parts of the Disney and Lang paper as possible, in a way that could be relatively easily - and convincingly - explained without the use of anything other than some simple algebra (and a big pile of definitions).

The toy galaxy ngc3314 introduced, which I gave the name T1, is the key, I think, to developing just such a thing. But it's going to need some work, and I'm inflicting that work on y'all, rather than do it all invisibly. :eek: :evil: :p

The colours aspect is an interesting spin-off; it will play a more important role than it has - so far - fairly soon (and you've already touched on several of the main points anyway!).



I think I was trying a defocusing technique to avoid over exposure in order to capture its "true" color. It does happen to be the color I see it when defocused through the scope, and at a high altitude.

I've seen that technique written up - Sky&Telescope?



I didn't know this. This would mean, I assume, that the shroud of gas around these galaxies has all but elimiated the other portions of the spectrum? If so, they must be quite dim, so even if it was OIII, it would be too dim to be seen and appreciated as green. I hear it is tough being green. ;)


Check out the spectrum of SDSS J113902.01+310336.7 (http://cas.sdss.org/astro/en/tools/explore/obj.asp?id=587739646210998382) (click on the GIF image of the spectrum at the bottom of the page). In this case the continuum is close to zero, and the Balmer lines relatively weak (though why H-beta is almost as strong as H-alpha is a mystery to me). Now move this around (increase and decrease the redshift), so the [OIII]5007/4959+H-beta complex moves into different parts of the colour spectrum ...

SDSS J110116.39+004814.5 (http://cas.sdss.org/astro/en/tools/explore/obj.asp?id=588848900980015266) is the same sort of thing, but with different relative strengths of the strong emission lines (the two [OIII] lines are more dominant here). To be sure, both these galaxies are small (the Petrosian radius is ~the seeing!), ...

(In case any reader is interested, these are "Green Peas"WP (http://en.wikipedia.org/wiki/Pea_galaxy), a hithertofore unknown class of galaxy discovered by zooites; AFAIK, several hundred have been found).

Now what if a Green Pea (or similar) galaxy has a very strong [OII]3728 line, and a redshift which puts the other major emission lines beyond the red end of human vision?

ETA: added link to Wikipedia entry on Green Peas

Nereid
2012-Feb-04, 10:20 AM
I'm going to standardise my criteria.

In the NDTSS (Nereid Digital Toy Sky Survey), a toy galaxy is detectible, in a toy band (did I really just write that?!? :rolleyes:), if its integrated magnitude is <22.2 in that band AND if its surface brightness (SB) is <25.0 (magnitudes per arcsec squared) in that band.

The next bit is tricky, but crucial; I need a robust way to switch between energy (per second per square centimetre, so it's actually flux) - which is what the Tolman signal is defined in - and magnitudes, which is what astronomers (including toy ones) work in1. Here's what I've chosen2:

I work in the STMAG system, of monochromatic (apparent) magnitudes per unit wavelength; i.e. I need a way to deal with spectral flux densities, and this is how I've done it. Here's the conversion:

m_\lambda\equiv-2.5log_{10}f_\lambda-21.1

Why?

Well, I need to add energies (actually fluxes), because my toy galaxies are colourful, they emit light in more than one wavelength (if they were monochromatic, I could work entirely in bolometric magnitudes ... which is what I did, up to T7).

Here's an example: T8. This toy galaxy is composed of five toy galaxies combined into one. Each of these five is monochromatic, but each has a different Mbol. What, then, is Mbol of T8? Well, use the above conversion formula (actually a definition) to get the flux densities of each component, add those flux densities, then convert back to magnitudes!

So, -17.7 (Mbol of the 265.0 nm component) is 0.0437 (ignoring units); -19.7 (the 350 nm component) 0.275; -21.3 (460.0 nm) 1.2; -21.9 (558.3 nm) 2.09; and -22.3 (666.5 nm) 3.02. Adding them up, we get 6.6287; putting that back into the formula we find that T8 has an Mbol of -23.15.

Yes, I also need to standarise rounding and precision; generally, magnitudes will be given to one decimal place, and ▒0.1 mag will be good enough.

1 "An ancient and arcane, but compact and by now unchangeable, way of expressing brightnesses of astronomical sources", as one source puts it (follow link in this earlier post in this thread (http://www.bautforum.com/showthread.php/120972-Bias-effects-in-galaxy-detection?p=1947004#post1947004), it's on p10)
2 If this is unclear, going to be unwieldly, or is downright wrong, I'd appreciate you, dear reader, saying so (my skin is pretty thick, so you can be as blunt as this forum's civility rules allow).

ngc3314
2012-Feb-04, 02:13 PM
I didn't know this. This would mean, I assume, that the shroud of gas around these galaxies has all but elimiated the other portions of the spectrum? If so, they must be quite dim, so even if it was OIII, it would be too dim to be seen and appreciated as green. I hear it is tough being green. ;)

It can also happen when the stellar population is so young that the great majority of its direct radiation is produced in the UV; most of the (large) fraction of that shortward of the Lyman limit at 912 A is absorbed and reprocessed into emission lines, some of which we see projected against the much weaker continuum in the optical bands. The equivalent widths [1] of emission lines, especially the hydrogen recombination lines, can be seen as measuring the slope of the continuum between their wavelengths as the wavelengths where the radiation is absorbed to ionize the gas.


False coloring is almost always better. I saw a true color for the "Pillars of Creation" and the result is rather a sickly red and orange columnated blobs. Hester et. al. false coloring allows for excellent revelations of the distinctive primary gases, with the side benefit of looking great.

Yeah, despite my rants about the Hester "Hubble palette", using energy-based color balance and the closest visual approximations for strong emission lines, most nebulae just look like assorted hues of crimson. The Hubble palette does a much better job of making changes i line ratios (and thus physical conditions) clear on visual inspection, aside from being visually very striking and sort of appealing.

[1] Equivalent width is a normalized expression of the flux (positive or negative) in a spectral line - the definition is the wavelength span of a stretch of the continuum with the same flux as the spectral line has. For an absroption line, this is the usually the only sensible way to measure its total impact.

George
2012-Feb-04, 05:11 PM
I can't say what others who are active here are trying to accomplish; however, I would like to demolish as many of the key parts of the Disney and Lang paper as possible, in a way that could be relatively easily - and convincingly - explained without the use of anything other than some simple algebra (and a big pile of definitions).

The toy galaxy ngc3314 introduced, which I gave the name T1, is the key, I think, to developing just such a thing. But it's going to need some work, and I'm inflicting that work on y'all, rather than do it all invisibly. :eek: :evil: :p If time allows, I'll try to be of a little more help.


The colours aspect is an interesting spin-off; it will play a more important role than it has - so far - fairly soon (and you've already touched on several of the main points anyway!).
Hornblower just reminded me of Roger Clark's work (http://www.clarkvision.com/visastro/omva1/index.html). He does a great job with visual astronomy.

...must run

Nereid
2012-Feb-04, 08:21 PM
I'm going to standardise my criteria.

In the NDTSS (Nereid Digital Toy Sky Survey), a toy galaxy is detectible, in a toy band (did I really just write that?!? :rolleyes:), if its integrated magnitude is <22.2 in that band AND if its surface brightness (SB) is <25.0 (magnitudes per arcsec squared) in that band.

I missed one: when is an object a point source, and when is it an extended source (i.e. when can we tell, by looking at an image of it, that it's a galaxy, and not a star/quasar)?

Simple: if it's bigger than the FWHM seeing, it's an extended source/galaxy! In the case of the NDTSS, the FWHM seeing is 1.4".

Oh, and for convenience, "local" means "at z = 0.01" (unless otherwise specified).

Nereid
2012-Feb-04, 08:48 PM
The next bit is tricky, but crucial; I need a robust way to switch between energy (per second per square centimetre, so it's actually flux) - which is what the Tolman signal is defined in - and magnitudes, which is what astronomers (including toy ones) work in1. Here's what I've chosen2:

I work in the STMAG system, of monochromatic (apparent) magnitudes per unit wavelength; i.e. I need a way to deal with spectral flux densities, and this is how I've done it. Here's the conversion:

m_\lambda\equiv-2.5log_{10}f_\lambda-21.1

Why?

Well, I need to add energies (actually fluxes), because my toy galaxies are colourful, they emit light in more than one wavelength (if they were monochromatic, I could work entirely in bolometric magnitudes ... which is what I did, up to T7).

Here's an example: T8. This toy galaxy is composed of five toy galaxies combined into one. Each of these five is monochromatic, but each has a different Mbol. What, then, is Mbol of T8? Well, use the above conversion formula (actually a definition) to get the flux densities of each component, add those flux densities, then convert back to magnitudes!

So, -17.7 (Mbol of the 265.0 nm component) is 0.0437 (ignoring units); -19.7 (the 350 nm component) 0.275; -21.3 (460.0 nm) 1.2; -21.9 (558.3 nm) 2.09; and -22.3 (666.5 nm) 3.02. Adding them up, we get 6.6287; putting that back into the formula we find that T8 has an Mbol of -23.15.

Yes, I also need to standarise rounding and precision; generally, magnitudes will be given to one decimal place, and ▒0.1 mag will be good enough.

1 "An ancient and arcane, but compact and by now unchangeable, way of expressing brightnesses of astronomical sources", as one source puts it (follow link in this earlier post in this thread (http://www.bautforum.com/showthread.php/120972-Bias-effects-in-galaxy-detection?p=1947004#post1947004), it's on p10)
2 If this is unclear, going to be unwieldly, or is downright wrong, I'd appreciate you, dear reader, saying so (my skin is pretty thick, so you can be as blunt as this forum's civility rules allow).

There may be circumstances in which the integrated apparent magnitude, across all ten NDTSS bands, is important. So far, however, that's purely cosmetic (testing is done by detection and classification in one or more bands).


As promised, the 'visibility' of the toy galaxy, T8, in the revised toy filter/bands, at various redshifts.

Recall that T8 has an Mbol of -23.1, and emits all its light in five lines - at restframe wavelengths of 265, 350, 460, 558.3, and 666.5 nm.

Locally, and at z=0.01, T8 would be visible in just four of the toy bands: nt2, ut, gt, and rt (visible in the sense that light emitted by it could, potentially, be detected; whether it would actually be detected depends - obviously - on details of the toy telescope+photometer we use to observe it). Note that the emission at 558.3 and 666.5 nm would both be detected in the rt band.

Here's one place I may need to be confident I'm doing integrations correctly.

At z = 0.01, what's observed in the rt band comes from T8's emission at both 558.3 and 666.5 nm (restframe); using the above method, T8 has an apparent rt magnitude of 10.28, which leads to an estimated Mbol of -22.87; its observed SB would be 20 mag/arcsec^2 (in the rt band).

Nereid
2012-Feb-04, 08:50 PM
Hornblower just reminded me of Roger Clark's work (http://www.clarkvision.com/visastro/omva1/index.html). He does a great job with visual astronomy.


That is awesome! :clap:

George
2012-Feb-06, 09:31 PM
I missed one: when is an object a point source, and when is it an extended source (i.e. when can we tell, by looking at an image of it, that it's a galaxy, and not a star/quasar)? If it is reasonably bright, it might appear as an extended object. To test if it is, couldn't we use greater magnification to see whether or not it increases in apparent size?


Simple: if it's bigger than the FWHM seeing, it's an extended source/galaxy! In the case of the NDTSS, the FWHM seeing is 1.4". I am unclear if this applies so nicely as it seems on paper, however. If the point source is relatively bright, it will appear extended, and the measuring may be a bit tricky for FWHM -- not that I have ever messed with this, admittedly. Just asking.

parejkoj
2012-Feb-06, 10:29 PM
Can I answer?


If it is reasonably bright, it might appear as an extended object. To test if it is, couldn't we use greater magnification to see whether or not it increases in apparent size?

I don't know the cost of the telescope used in the NDTSS, but most surveys that get good signal-to-noise at ~20th magnitude and 1.4" seeing have costs in the tens of millions of dollars. "Changing the magnification" means building another telescope, in a location with better seeing.

Sorry.

Nereid: I haven't seen a proper cost writeup for the NDTSS? What kind of construction costs are you looking at? :)


I am unclear if this applies so nicely as it seems on paper, however. If the point source is relatively bright, it will appear extended, and the measuring may be a bit tricky for FWHM -- not that I have ever messed with this, admittedly. Just asking.

Stars, being point sources, have a roughly Gaussian light profile. Specifically, a point source has a light profile that is exactly the point spread function (PSF) of the optical system + seeing. Most telescopes over-resolve the PSF, so that the light from a point source falls across several pixels (usually ~2-3 pixels in each direction). If the PSF is well modeled, though a combination of proper understanding of the optics plus measurements of the profiles of known, moderately luminous, unsaturated stars, you can say that anything with a light profile "similar enough" to the PSF is a point source, and thus a star (or quasar). Anything that isn't fit well by the PSF is not a point source.

This becomes very tricky for very faint, very distant sources, where the difference between a PSF fit and a "galaxy-like" fit is statistically indistinguishably. There is a limit in any image where you can't tell the difference between a faint point source and a small, faint galaxy. SDSS III is already working at that limit for its photometric data, and has to use object colors and a map of the density of known stars to help disentangle these. It still sometimes gets it wrong, as when an object is targeted for spectroscopy because it is thought to be a z~0.6 galaxy, but it turns out to be a star. There's a whole paper about correcting some of these systematics (http://adsabs.harvard.edu/abs/2011MNRAS.417.1350R), as applied to the SDSS III data.

George
2012-Feb-07, 09:03 PM
Thanks for the explanation, parejkoj.

Nereid
2012-Feb-08, 05:24 PM
Introducing T10.

Like T8, and all the other toy galaxies to date (except T9), it is a uniformly luminous disk; unlike them, its radius is 3.2 kpc.

Like T8, it emits only at the (restframe) wavelengths 265, 350, 460, 558.3, and 666.5 nm.

If we regard each line emission as a separate galaxy, the Mbol's would be -17.2, -19.4, -21.1, -21.7, and -22.2, respectively. Combined, this gives T10 an Mbol of -23.

By my criteria, T10 becomes indistinguishable from a point source somewhere between z = 0.25 and 0.5.

In terms of detectability, it would be pretty obvious locally - in each of the n2t, ut, gt, rt, and it bands - locally and at z = 0.1. It would have 'sunk' in the ut band by z = 0.25, and the gt band by z = 0.5 (i.e. it'd be a point source in only three bands, it, zt, and IR1t). This would become just two bands at z ~1, IR1t, and IR2t. Somewhere between z = 1 and z = 1.5, T10 would sink entirely.

If, somehow, we could observe T10 across all 10 toy bands in an integrated fashion, it would be barely detectable at z = 1.5, and have sunk at higher redshifts.

Nereid
2012-Feb-08, 05:35 PM
Can I answer?

Of course you can!

And I see that you did, thanks. :clap:



I don't know the cost of the telescope used in the NDTSS, but most surveys that get good signal-to-noise at ~20th magnitude and 1.4" seeing have costs in the tens of millions of dollars. "Changing the magnification" means building another telescope, in a location with better seeing.

Sorry.

Nereid: I haven't seen a proper cost writeup for the NDTSS? What kind of construction costs are you looking at? :)

Fair question.

Let's see now ... PC, ~Ç1,500; software, free; internet connection, ~Ç50/month; time ... I'd rather not say. :p

Building telescopes, with snazzy filters and automated operations (e.g. observation scheduling, data reduction pipelines), in my toy universe is astonishingly cheap. :D However, I'm so happy that someone else has invested hundreds of millions (billions?) of $/ú/Ç in real telescopes, etc, and millions of hours of effort in writing up and publishing results. This has enabled me to build my toy survey - so far, just the NDTSS - in a way that rather nicely approximates a very real survey (SDSS). :dance: :)

Nereid
2012-Feb-08, 05:37 PM
Introducing T11.

Like T8, and all the other toy galaxies to date (except T9), it is a uniformly luminous disk; unlike them, its radius is 32 kpc.

Like T8 and T10, it emits only at the (restframe) wavelengths 265, 350, 460, 558.3, and 666.5 nm.

If we regard each line emission as a separate galaxy, the Mbol's would be -17.7, -19.7, -21.2, -21.7, and -22.1, respectively. Combined, this gives T11 an Mbol of -23.

Being much bigger than T8 and T10, T11 remains an extended source at all redshifts.

However, its large size means that its SB is low; so low in fact that it would not be detectable, even locally, in the n2t and ut bands. And in the gt band, it'd be an LSB (low surface brightness) galaxy even locally; by z=0.25 it'd be marginally undetectable in the rt band, but clearly detectable in both the it and zt bands.

By z = 0.5 it would be marginally detectable in only the IR1t band.

If we could observe it across all bands, it would be marginally detectable at z = 0.75, but would have sunk by z = 1.

Now suppose T8, T10, and T11 were, in fact, the same galaxy. In other words, an axisymmetric disk (in terms of uniformity of emission), with three sharp jumps in surface brightness, at radii of 3.2 kpc, 10 kpc, and 32 kpc. Stay tuned! :)

Nereid
2012-Feb-08, 08:15 PM
OK, I'm a tease.

In this post, instead of examining T12, the superposition - in some sense - of T8, T10, and T11, I'm going to look at three variants of T1, a red (T13), white (T14), and blue (T15) galaxy [1].

T14, the white galaxy.

Remember, it's a uniformly luminous disk, 10 kpc in radius, with an Mbol of -24. T14 emits light at ten discrete wavelengths: 153, 259.5, 366.3, 469.3, 604.3, 748.7, 903.7, 1085.5, 1318.8, and 1615 nm (restframe). Each Mtoyband is thus -21.5. As with T1, T14 is always an extended object in NDTSS, no matter what the redshift. By z=1, it has sunk, if observed by/in the NDTSS, in terms of per-band integrated magnitude ... and even if it hadn't, it's a dropout in the ut, rt, and IR1t bands (i.e. none of the lines gets redshifted into these bands). However, it doesn't disappear until a somewhat higher redshift (but below 1.5), in terms of falling below the SB limit.

T13, the red galaxy.

This galaxy is also a uniformly luminous disk, 10 kpc in radius, with an Mbol of -24. Like T14, it emits light at the same ten discrete wavelengths. However, the ratio of flux at any two such is 1.2, with the longer wavelength having the greater flux. For this galaxy, the 'disappears in integrated magnitude in a given band before disappearing, in the same band, because the SB drops too low' (whew) is the same, as for T1, T2, and T14. However, the redshift at which it sinks is different: the shortest wavelength emission (restframe 153 nm) - which by then is observed in the n2t band - has sunk before z=1. It's still detectable in the next (259.5 nm), in the gt band, at this redshift, but is gone by z=1.5, It's the same for all the other emission lines, except the last four. However, for them, redshift has taken them beyond the IR3t band, so they won't be detected.

T15, the blue galaxy.

Ditto, except that the 1.2 ratio is for the flux at the shorter wavelength of each pair. Applying the same test (i.e. looking only at when the SB falls too low), T15 sinks only at a redshift a tad above z=2, at the shortest wavelength (where it's detectable in the gt band). Ditto for the next longest emission line (iz band). At longer wavelengths, the galaxy sinks well before the SB limit is reached, because these wavelengths are redshifted beyond the IR3t band. In other words, despite having the exact same Mbol, T15 remains detectable until (somewhat after) z=2, as a galaxy, whereas T14 (the white galaxy) and T13 (the red galaxy) have sunk by z=1.5.

But how realistic are these red, white, and blue galaxies? Not particularly. However, if you don't know what the restframe SED of a galaxy is, outside the tiny sliver of the electromagnetic spectrum astronomers doing surveys in the 1960s were limited to (UBVR, to summarise), the Disney&Lang analysis is severely limited.

In terms of flux, a galaxy will usually have an unambiguous - if sometimes rather broad - maximum in its SED. After all, two of the three main components of real galaxies - stars, gas/plasma, and dust (AGNs are ignored in this analysis) - can be crudely approximated by Plankian SEDs (gas/plasma is the exception). However, those maxima can certainly be outside the 'optical' (i.e. UBVR); a galaxy with lots of young, bright blue stars may have its SED's maximum in the UV, and a dust-choked one, well into the MIR.

In fact, each of T8, T10, and T11 have SEDs that are still rising at the red end. :eek:

OK, can we look at T12 now? Stay tuned! :)

[1] No, I'm not necessarily feeling love for the USofA; do you know how many of the world's flags contain red, white, and blue (and only red, white, and blue)? Sticking to only major ones, and going alphabetically, Australia, Cambodia, Chile, Cuba, Czech Republic, ...

Nereid
2012-Feb-08, 09:37 PM
OK, the tease continues. :p

In this post, instead of examining T12, the superposition - in some sense - of T8, T10, and T11, I'm going to introduce you to a marvelous camera, the TACS (toy ACS, get it?), which is mounted on an awesome platform, the THST.

The TACS is equipped with ten filters, as earlier, and the TACS+THST combo has wonderful optics. And, being out of this world (i.e. above the Earth's atmosphere) its PSF is a mere 0.05" (FWHM)! Using this facility, galaxies become undetectable - as point sources - at 26.5 mag, in all bands (vs 22.2 in the NDTSS). In terms of SB, this facility is better than the NDTSS too; the cutoff is 27 mag per arcsec squared, in all bands (vs 25 in the NDTSS).

How do T8, T10, and T11 fare, when observed by TACS+THST? What about T13, T14, and T15?

Well, T10 never becomes confused with a point source; all galaxies are easily resolved as extended sources.

The white galaxy (T14) sinks around (a bit before) z=3, and the blue (T15) a tad beyond z=4; in both cases the SB falls below 27. And the red (T13)? Somewhere between z=2 and z=3. As with being observed in the NDTSS, the problem is that the wavelengths in which T13 is brightest have been redshifted beyond the IR3t band, so the galaxy sinks below the SB limit more quickly. In terms of integrated magnitude, within a band, the red and white galaxies disappear somewhere between z=5 and 6; the blue galaxy remains easily detectable at z=6.

T11. Its low SB means that it will sink at quite modest redshift, cosmologically speaking. Even locally, it would be an LSB-too-farfaint in the n2t band (though its integrated magnitude would make it quite impressive). In whatever toyband the emission lines were redshifted to, T11 would be detectable at higher z than in the NDTSS, out to z=1.5 (well, not quite), in the IR3t (or perhaps IR2t) band.

T8: Easily detectable to z=2; sunk by z=3, mostly because the brightest emission lines have redshifted beyond the IR3t band.

T10: This galaxy's greater SB, in all toybands, means that it's much more easily detectable. Even in the shortest wavelength emission line - redshifted to the gt band - it's easily detectable at z=0.75 (but has sunk at z=1). At the next line (350 nm), its SB does not lead it to sink until z=3 (although its integrated magnitude is below the 26.5 limit). Beyond this it disappears because the brighter lines have been redshifted beyond the IR3t band; if the TACS were equipped with a suitable MIR (?) photometer, T10 would be easily detectable at z=6.

In summary, the TACS+THST is certainly able to detect these toy galaxies at cosmological distances, some of them almost to z=5 or 6.


The Visibility Window depicted in fig 3 is immutable, mathematical and pinned in local coordinates because it shows the contrast to ones local sky, be it on the ground or in space. What we need to calculate next are the properties, in particular the sizes and intrinsic SBs, of the kinds of galaxies, seen at different redshifts, which will make it through that narrow window, particularly near its peak, taking into account the Tolman e ects described above, which both dim a galaxy and increase its apparent size.

[...]

Even at z = 0.5 many of the most Visible galaxies that were in region A (Fig 1) at low redshift would be translated into region C and be far too dim to see. They have Sunk.

[..]

More than 50 per cent of the light from a galaxy that would be at the peak nearby, has already been lost at redshift 0.5, 82 per cent at redshift 1, and all by 1.2. These figures alone are enough to query the feasibility of trying to study galaxy evolution by using deep fields.

[..]

If now we remove the Local population to redshift 1, virtually all the previously prominent galaxies will sink below the sky thanks to Eqn. (43).

At least in the toy universe in which I've been doing my observing, what Disney & Lang write is, not to put too fine a point on it, hogwash.

So, you must be thinking, how closely does my toy universe resemble the real one?

But first, can we look at T12 now, please? OK, sure thing, in my next post ...

Nereid
2012-Feb-08, 10:04 PM
I've promised to look at T12, the superposition - in some sense - of T8 (a red, uniform, 10 kpc disk, Mbol -22.9), T10 (a red, uniform, 3.2 kpc disk, Mbol -23), and T11 (a red, uniform, 32 kpc disk, Mbol also -23).

First, though, I'd like to point out some things mildly interesting about these three galaxies. Despite their very different sizes, they have pretty much the same Mbol, each a full magnitude fainter than T1. Their colours are similar, but not the same. Let's observe them at z=0.25, so each of the five emission lines falls in just one toy band, and those bands somewhat resemble the SDSS ones (for brevity I've shortened the band names; the first colour should be ut-gt).


Tn. u-g g-r r-i i-z
T08 2.0 1.6 0.6 0.4
T10 2.2 1.7 0.6 0.5
T11 2.0 1.5 0.5 0.4

OK, has a lightbulb gone off in anyone's head yet?

T12 is built from T8, T10, and T11 as follows: the core is T10. Cut out a disk of radius 3.2 kpc from T8, so that it's now an annulus. Plonk this down on top of T10, so the centres coincide (all disks are in the same plane). Cut out a disk of radius 10 kpc from T11, so it too is now an annulus. Plonk it down on top of T10+T8' (i.e. the modified T8). Anyone want to have a go at saying how visible T12 would be, in the NDTSS? In a survey taken with the TACS+THST?

Now I had planned to repeat this exercise, with modifications/extensions of T1 (or T13, T14, and T15), building up to my grand finale, ... T31 (get it?). Along the way I'd have done a slow reveal for T12, and any extensions etc necessary to make T31 seem like an amazing coincidence. :D :p :eek:

However something funny happened on the way to the forum before I could get very far.

What? Well, a dog failed to bark in the night. :confused:

Specifically, an interesting paper/preprint did not get mentioned (http://www.bautforum.com/showthread.php/123740-Fun-Papers-In-Arxiv). ;)

But that story is going to have to wait, until another day ...

Nereid
2012-Feb-09, 04:07 PM
Just in case any reader is interested in the how of my toy universe observing (are you such a reader? please speak up if you are!), here are some details.

I particularly like ngc3314's approach, as I've already said.

First, it starts with the total energy flux, across the entire electromagnetic spectrum. This is what the Tolman dimming is keyed to, so the physics is sound.

Next, the uniformly luminous circular disk, normal to our line of sight, is both an easily understood thing (element), and one that can be easily and robustly manipulated. I call it an element because it acts like a Lego block; you can readily add lots together to make realistic models. For example, as I started to do with T12, a series of nested disks can approximate an exponential SB profile, or a de Vaucouleurs one (β=4), or any SÚrsic profile, or ... and this can be done to essentially an arbitrary degree of detail.

Independently, the SEDs of the disk elements can be approximated by discrete emission lines, chosen to fall just where you want them in terms of the photometry you want, or plan, to do. Because the disks' total emission - energy flux - is what you start with, allocating this across bands also gives you colours, for free as it were. Sure, it's a real pain to have to be constantly switching back and forth between energy/power (e.g. joules/watts) and magnitudes (AB system? STMAG system? have I messed up my zero points? ...), and the chances that you'll mess up very high, but at least you can always see what's going on, in a (relatively) straight-forward and intuitive way.

The work I've done to date has been done in a spreadsheet. I'm sure it could be taken a lot further, especially with macros; however, I think it'd be a lot easier to switch to code. For one thing, it's surely a lot easier to go from three nested disks to 100 in code than it is in a spreadsheet; ditto ten emission lines to 1,000. And I'd planned on doing so, until I stumbled upon the dog which did not bark ...

Anyone yet worked out what real universe galaxy T12 approximates?

Nereid
2012-Feb-10, 07:42 PM
Here are some (edited) extracts from the preprint; I'll give full details in a later post.


Our sample of {galaxies} is drawn from the HST imaging with Advanced Camera for Surveys (ACS) and WFC3, which was obtained as part of the [...] program. Near-UV and near-IR observations were acquired as part of the WFC3 [...] program [...] a 104 orbit medium-depth survey using the HST UVIS and IR cameras. A general introduction to the performance and calibration of the WFC3 is provided in Windhorst et al. (2011). The [...] program observed approximately 50 square arcminutes in the GOODS-S field with the HST WFC3 UVIS in three filters: F225W and F275W for 2 orbits, and F336W for 1 orbit, per pointing, respectively. The program observed approximately 40 square arcminutes in the same field with the WFC3 IR in three filters: F098M, F125W, and F160W, each for 2 orbits per pointing. The 5σ 50% point-source completeness limits are: F225W=26.3, F275W=26.4, F336W=26.1, F098M=27.2, F125W=27.5, and F160W=27.2 mag (see Windhorst et al. 2011). The analysis presented here was completed using mosaicked images produced for each of the UVIS and IR band tilings, and each image mosaic was drizzled to a pixel scale equal to 0.090" pixel-1.

[...]

The WFC3 mosaics roughly cover the northern one-third of the GOODS-S field (Giavalisco et al. 2004), and we incorporate the pre-existing ACS dataset (F435W, F606W, F775W, and F850LP) with the WFC3 observations. We produced mosaicked images of the GOODS-S ACS data, which were binned to match the pixel scale of the WFC3 UVIS/IR mosaics.

Selection Criteria

We require our galaxies to have: (1) been imaged in all UV and IR bands, to uniform depth; (2) a spectroscopically-confirmed redshift in the range 0.35 ~< z < ~1.5; and (3) an {galaxy} morphology.

There are many techniques for identifying {galaxies} at intermediate redshift. [...] However, the robustness of each of these classifiers can be dramatically affected by a variety of systematics, such as the image signal-to-noise ratio (Conselice et al. 2003; Lisker 2008) and the bandpass in which the technique is applied (Taylor-Mager et al. 2007; Conselice et al. 2008). In lieu of these techniques, we select our sample by visual classification. This technique is subjective, and as such can introduce new biases, but it has been successfully applied to the identification of both low redshift (z ∼0.1; [...]) and intermediate redshift (z ~< 1.3; [...]) {galaxies}. We will demonstrate in ž{Y} that the spectroscopic redshift requirement, and not the morphological selection technique, is the most significant source of bias.

[...]

UV imaging can provide unique insight into {something interesting}. Thus, we require our sample {galaxies} to be observed in each of the UV filter mosaics. To ensure that all galaxies were observed to a similar depth, we also require each {galaxy} in the sample to be observed in the UV and IR image mosaics for at least the mean exposure time measured for each filter as given by Windhorst et al. (2011). Since we are interested in {something interesting}, and the WFC3 UVIS channel is only sensitive to UV emission at  ∼ 1500 ┼ for objects at redshift z ~> 0.35, we define this redshift as low-redshift cutoff of the sample. The high-redshift cutoff was selected to ensure that the visual inspection and classification of the {galaxy} Ś in the filter set outlined above ľ considers the rest-frame V-band morphology. We are sensitive to at least the UV-optical SED of every {galaxy} in our catalog.

[...]

We find 102 {galaxies} that satisfy these selection criteria.

Photometry

We measured object fluxes using SExtractor in dual-image mode (Bertin & Arnouts 1996), with the WFC3 F160W image as the detection band. For source detection, we required sources to be detected in minimally four connected pixels, each at ≥ 0.75σ above the local computed sky-background. For deblending, we adopted a contrast parameter of 10−3 with 32 sub-thresholds. Object photometry was determined with MAG AUTO parameters Kron factor equal to 2.5 and minimum radius equal to 3.5 pixels.

We adopted gains for each filter using the mean exposure time calculated for each mosaic as follows: F225W and F275W equal to 5688 sec; F336W equal to 2778 sec and F098M, F125W, and F160W equal to 5017 sec (see Windhorst et al. 2011). From Kalirai et al. (2009a,b) we assumed zeropoints for the filter set F225W, F275W, F336W, F098M, F125W, F160W equal to 24.06, 24.14, 24.64, 25.68, 26.25, 25.96 mag, respectively. We assumed zeropoints for the filter set F435W, F606W, F775W, and F850LP equal to 25.673, 26.486, 25.654, and 24.862 mag, respectively.

In Table {X} we present the measured photometry for the {galaxies}. SExtractor non-detections are designated " Ě Ě Ě " (23 galaxies) and {galaxy} fluxes with detections fainter than the recovery limits (discussed below) are designated "Ś" (52 galaxies), as explained in the footnotes of Table {X}.

The combination of the stable WFC3 UV-optical-IR PSF and high spatial resolution allows many compact or low surface brightness (SB) {galaxy} candidates to be detected and measured. These candidates may meet the morphological selection criteria in the "detection" image, but in dual-image mode SExtractor returns flux measurements for these {galaxies} which are significantly below the formal completeness limits in the "measurement" image. Their formal flux uncertainties are larger than ∼1 mag (implying a signal-to-noise ratio ~< 1). To ascertain the reliability of these faint flux measurements in the UV bandpasses, we inserted simulated galaxies into the images, and performed an object recovery test to measure the flux level where the signal-to-noise typically approaches ∼1. To derive 90% confidence limits, we inserted ∼60,000 simulated galaxy images representing a range of total magnitudes (24 mag < m < 30 mag) and half-light radii (0.8" < rhl < 2.25") into each of the UVIS mosaics, and measured the fraction of simulated galaxies which were recovered by SExtractor, using the same SExtractor configuration as discussed above. The simulated galaxies were defined with an r1/4 ("bulge") or exponential SB profile (ü"disk"). From these simulations, we estimated the 90% recovery limits for simulated [...] profiles with half-light radius of 1.0" equal to F225W=26.5, F275W=26.6, F336W=26.4, and F435W=26.7 mag, respectively. We interpret {galaxies} with magnitudes fainter than these recovery limits as 1-σ upper limits.

Let's remind ourselves of one of Disney & Lang's most eye-catching lines: "If now we remove the Local population to redshift 1, virtually all the previously prominent galaxies will sink below the sky thanks to Eqn. (43)."

Hmm.

You'll be able to see for yourself, later, from Table {X}, just how many of the 102 galaxies examined - in considerable detail - have a redshift of >=1. Too, once I tell you what {galaxies} this (as yet unnamed) paper sought to study, you can decide if any are prominent in the "Local population" (Disney & Lang - deliberately? - do not provide a definition of what they mean by this term).

In any case, I would guess that Disney & Lang would be rather astonished by this paper; in quite a few ways it would seem - at face value - to contradict several of the main conclusions of their own. And, as in all science, experiment (or observation) always trumps theory ...

Jerry
2012-Feb-10, 10:06 PM
I always get to wonder how much the real sinking of galaxies below Neried's horizon distorts our over-all perceptions. Consider the assumption that most ultra-bright uv events are more-or-less universally shrouded and highly directional. We would see very few of these locally, but the odds of finding the precise veiwing angle will increase with distance, and be doubled and redoubled by lensing effect. Neried's toy universe predicts very different fall-out patterns when complex geometries come into play.

Nereid
2012-Feb-11, 01:50 PM
I always get to wonder how much the real sinking of galaxies below Neried's horizon distorts our over-all perceptions. Consider the assumption that most ultra-bright uv events are more-or-less universally shrouded and highly directional. We would see very few of these locally, but the odds of finding the precise veiwing angle will increase with distance, and be doubled and redoubled by lensing effect. Neried's toy universe predicts very different fall-out patterns when complex geometries come into play.

Interesting comment Jrery.

As it's about "events", not galaxies, perhaps we could discuss it further, in a separate thread?

Let me return to a key theme in the D&L paper, prominence; e.g. "If now we remove the Local population to redshift 1, virtually all the previously prominent galaxies will sink below the sky thanks to Eqn. (43)."

Now, to me, what make local galaxies prominent isn't only their surface brightness in the B (e.g. ~386 to 485 nm) and V (e.g. ~490 to 584 nm) bands; it's also their size. Moving any local galaxy - that we know of - to redshift 1 will make it small. Take one that we can approximate as a disk of radius 100 kpc [1]; from at least z = 0.5 to z = 6, this galaxy will appear as a circle (if we view it face-on) of not even 25 'SDSS seeing-pixels' (in radius). Hardly 'prominent', eh?

It's pretty obvious that D&L do mean to imply that there are, locally, galaxies we are not aware of, with effective radii > 100kpc!

But if you take the (strongly implied) 'high SB, in the B and V bands' prominence criterion, it's easy to see why their paper is looking at the wrong thing. Consider the redshift at which the very bluest of detectable photons from a local galaxy sinks from detectability, by moving beyond the red end of the V band. What is that redshift? Why it's only 1.51. :eek: In other words, no galaxy we detect, in B and V, at redshifts > 1.5, is prominent locally (no matter how high its local B and/or V SB is)!

Oh, and Jeryr, it's Nereid.

[1] do you, dear reader, know any real galaxy, of any redshift, that is, in some sense, this big?

Nereid
2012-Feb-12, 06:19 PM
Continuing with some comments on prominence.


Or could they be representatives of quite a different dynasty whose descendents are no longer prominent today? We explore this latter hypothesis and argue that Surface Brightness Selection E ects naturally bring into focus quite di erent dynasties from different redshifts. Thus the HST z = 7 galaxies could be examples of galaxies whose ...

For starters, as has already been noted earlier in this thread, "the HST z = 7 galaxies", observed in the (observer) UBV frame would not only not be prominent locally (i.e. if magically transported as they are/were to z = 0), but they'd be completely invisible (or nearly so)!

Why?

Because what we see of these galaxies, now, at z~7, in the UBV bands, is/was emitted in the EUV, blue-ward of the Lyman limit ... and we see precisely zero such galaxies locally (http://www.bautforum.com/showthread.php/128354-Galaxies-observed-in-the-EUV) (thanks ngc3314).

And even at lower redshifts, what are the objects which are prominent locally that we would expect to see, in UBV (at a redshift of, say, 1 - 3)?

Well, they would certainly include the Markarian galaxies (http://www.aras.am/Dfbs/markariangalaxies.htm).

The most prominent local galaxies (other than the MW), in the UBV bands, are, of course, the Magellanic Clouds, M31, and M33. The first two are dwarfs, and are only prominent because they are so close. And M33 isn't much bigger, or brighter. M31 and the MW? Well, they're not Markarian galaxies, no matter how much you stretch the definition of such galaxies. If we widen our definition - of locally prominent galaxies - to the Messier objects galaxies, how many of those are (northern) Markarian galaxies?

By (deliberately?) ignoring this 'prominent in one narrow slice of the spectrum' != 'prominent in another, quite different, slice', D&L have thus rendered most of their paper relevant only to a toy universe which is dramatically different from the real one we do our astronomical observations in.

Nereid
2012-Feb-13, 05:35 PM
As is clear from the various posts in this Q&A thread (http://www.bautforum.com/showthread.php/128414-If-Messier-had-travelled-to-Australia-...), prominence - as in, of galaxies - is a somewhat slippery concept.

Of interest, from that thread, to a broader question (than the ones D&L asked): there are lots of prominent galaxies in clusters (among the Messier galaxies, >40% are in the Virgo cluster ... and the non-Virgo cluster galaxies include four in the Local Group), and quite a few in tight groups.

If all the Messier galaxies were at the same distance from us - 10 Mpc, say - how would they be ranked, in terms of prominence?

I'll have a go at answering this later (unless someone beats me to it), but my guess would be that the giant ellipticals/lenticulars - which go by the moniker ETG (early type galaxies) I believe - would all be at, or near, the top; e.g. M84, M85, M86, and M87.

Nereid
2012-Feb-15, 05:41 PM
Well, the Messier galaxies are quite an interesting lot, when studied in terms of prominence! :)

First, the source I used: SEDS (http://messier.seds.org/objects.html).

If you plot the listed magnitude against distance, and also against size, fit a trend line (taking logs as appropriate), then identify the extreme galaxies (i.e. those which are farthest from the respective trend lines), what do you find?

Well, would you believe M31 is clearly the most extreme galaxy? :eek: :p

It is a full 3 mags brighter than it 'should' be, for its distance (the next most extreme, in this sense, is a mere 1.9 mags brighter), and 1.2 mags brighter than it 'should' be for its area - in square arcmins - which puts it equal second (the most extreme galaxy is 1.4 mags off the trend line).

Three other spirals are also significantly brighter by both criteria: M104 (the real outlier), M81 (still >1σ off in both), and M83 (<1σ in both).

In the faintness stakes, the local dwarfs (M32 and M110) take the 'distance' prize (i.e. their brightnesses are the furthest from the trend line), with M110 also being a wimp in the area race (M32 is well above that trend line). Several spirals are near, or over, the 1σ mark in both distance and area relationships: M74, M98, M91.

So which Messier galaxies are likely to stand out, if moved to greater and greater distances, ignoring any SED effects? If all you had to go on were the two relationships above, then they'd be the big, bright spirals, M31, M104, M81, and M83. Next would come some of the Virgo cluster ETGs, M49, M87, M86, and M60, and one of the Virgo spirals, M77. Spirals M33 and M51 would also be selected. Notably different is M101: ~1σ bright in the distance race, ~1σ faint in the area one.

And which galaxies best represent the two trends - as in, they are close to both trend lines? One spiral, M96, and one Virgo cluster ETG, M59.

But isn't it rather odd that so many of the closest galaxies are near, or at, the top of the class? Surely we don't, here in the MW, happen to be especially blessed with having the most prominent galaxy by far (M31) - out to a distance of ~60 Mly - right on our very doorstep? No, of course not. So, how to get a better handle on 'prominence'? Well, what would M31 (etc) look like if it were at a distance of 60 Mly?

Stay tuned! :D

StupendousMan
2012-Feb-15, 10:34 PM
Could you show us your graphs, please?

Nereid
2012-Feb-16, 12:34 AM
Could you show us your graphs, please?

I'd love to, but I'm not sure I can, easily.

However, here is the data (from SEDS):

31 S 2.9 3.4 178x63
32 E 2.9 8.1 8x6
33 S 3 5.7 73x45
49 E 60 8.4 9x7.5
51 S 37 8.4 11x7
58 S 60 9.7 5.5x4.5
59 E 60 9.6 5x3.5
60 E 60 8.9 7x6
61 S 60 9.7 6x5.5
63 S 37 8.6 10x6
64 S 19 8.5 9.3x5.4
65 S 35 9.3 8x1.5
66 S 35 8.9 8x2.5
74 S 35 9.4 10.2x9.5
77 S 60 8.9 7x6
81 S 12 6.9 21x10
82 I 12 8.4 9x4
83 S 15 7.6 11x10
84 E 60 9.1 5x5
85 E 60 9.1 7.1x5.2
86 E 60 8.9 7.5x5.5
87 E 60 8.7 7x7
88 S 60 9.6 7x4
89 E 60 9.8 4x4
90 S 60 9.5 9.5x4.5
91 S 60 10.2 5.4x4.4
94 S 14.5 8.2 7x3
95 S 38 9.7 4.4x3.3
96 S 38 9.2 6x4
98 S 60 10.1 9.5x3.2
99 S 60 9.9 5.4x4.8
100 S 60 9.3 7x6
101 S 27 7.9 22x22
102 E 45 9.9 5.2x2.3
104 S 50 8 9x4
105 E 38 9.3 2x2
106 S 25 8.4 19x8
108 S 45 10 8x1.5
109 S 55 9.8 7x4
110 E 2.9 8.5 17x10

Columns:
1: Messier number
2: galaxy type, S=spiral, E=ETG (i.e. elliptical or S0/lenticular), I=irregular
3: distance, in Mly
4: brightness, in mags
5: area, in arcmins

The fitted trend lines are:
Distance trend line: mag = 1.006806 * ln(dist) + 5.313
Area trend line: mag = -0.76188 * ln (area) + 11.7323

I think it should be easy to plot both graphs, from this data. If you do, and are able to post here, would you (or any other reader) please do so?

StupendousMan
2012-Feb-16, 04:04 AM
Using data from the list Nereid provided in the previous post, I computed the distance modulus for each galaxy, and the absolute magnitude of each galaxy.

16294

I do not see the trend that Nereid mentions. What I do see is a selection effect: we can and do see intrinsically dim galaxies nearby, but we don't see similar galaxies at large distances (because they don't reach our detection thresholds).

Nereid, if I've misinterpreted your post, please correct me.

Nereid
2012-Feb-16, 03:22 PM
Using data from the list Nereid provided in the previous post, I computed the distance modulus for each galaxy, and the absolute magnitude of each galaxy.

16294

I do not see the trend that Nereid mentions. What I do see is a selection effect: we can and do see intrinsically dim galaxies nearby, but we don't see similar galaxies at large distances (because they don't reach our detection thresholds).

Nereid, if I've misinterpreted your post, please correct me.

Yes, sorry; I wasn't as clear as I should have been (obviously).

What I am (was) doing is setting the scene for something similar to what you have, in fact, just done.

The context is (still, after all this time) the D&L paper. Specifically, I'm exploring one aspect of this statement therein: "If now we remove the Local population to redshift 1, virtually all the previously prominent galaxies will sink below the sky ...". Now D&L did not define what they meant by "the Local population", so I'm attempting to do so (note that I've already shown that their "remove ... to redshift 1" thought experiment to be seriously flawed, at least in terms of the conclusion they draw).

The Messier galaxies appear prominent. No argument with that, right? Further, since we (now) know they are all closer than ~20 Mpc, we can treat them as "the Local population" (or part thereof), at least as an exercise.

All I did was plot, on the y-axis, the observed mags (per SEDS), and on the x-axis distance (again, per SEDS). To get a nice-looking linear trend, I made the scale on the x-axis logarithmic*; add a trend line, and voilÓ! The mag-area relation is the same, with the extra step of calculating the area first.

Two points on the mag-distance plot/graph/chart (all these words all the same, to all my readers?) will pin down that trend line: (2.9,6.4), and (60,9.4).

By 'moving' all the Messier galaxies to 60 Mly (say), it will be possible to examine the question of what (sorts of) galaxies are (locally) prominent in an unbiased way^ ...

I'm pretty sure this sort of thing is so familiar to you, and other professional astronomers, as to be downright boring (so, many thanks for continuing to even read my posts in this thread); however, I suspect it's poorly understood by many BAUTians. In any case, by moving only in baby steps, I hope the general reader will not lose the plot (so to speak) in my demolition discussion of the D&L paper.

* the software I'm using enables me to do with by simply ticking a box; it can also produce a trend line - several actually, of different kinds - with the click of my mouse. Of course, I can do all this myself, successfully (modulo mistakes), but it takes me a lot longer ...

^ or at least allow examination of possible biases other than distance!

Nereid
2012-Feb-16, 08:03 PM
Repeating Stupendous Man's calculations, and plotting, I get the same as he does [1]. To recap: what this calculation of absolute magnitude, of each galaxy, does is put them all at the same distance (in this case 10 pc, so the numbers are ridiculous, but the conclusions are robust).

And what a difference removing that selection bias makes! In the top quintile (i.e. the top eight galaxies), there are now six ETGs and only two spirals ... that's half the ETGs of the entire set, in just the top quintile! The two spiral stars are M104 and M77; only 0.7 mags separates #1 (M104) from #8 (M85). The galaxy which is so dominant in the local sky - M31 - is now merely average (it's ranked #20). The difference between #1 and #20 is only ~1.6 mags.

So now we know something about what the locally prominent galaxies are, in terms of absolute brightness.

What about in terms of surface brightness?

Stay tuned!

[1] correcting for the fact that the SEDS distances are quoted in Mly, and SM took them to be Mpc.

Nereid
2012-Feb-17, 06:12 PM
The average surface brightness (SB)* of the Messier galaxies doesn't vary all that much; 19.7 (M105) to 23.5 (M101) mags per arcsec squared. The median SB is ~21.85, with most of the ETGs above (i.e. higher/brighter average SB) this (nine, out of 12). The one irregular (M82) makes it into the top quintile. There's no trend worth reporting, with distance.

However, all but one of the Virgo cluster spirals (M77) has a lower (fainter) average SB than any ETG, and only two of the ETGs have lower average SBs than M77 (M49 and M85).

It's looking like ETGs are, as a class, the winners in the prominence stakes, although an occasional spiral or two may win the prize of most prominent individual galaxy.

What of M104, the brightest Messier galaxy? It's in the top quintile, so it's certainly prominent (only M94 has a higher SB, among the spirals; a high SB dwarf isn't going to figure in questions about D&L's paper). And M31? It's quite a wimp; it ranks #32, just missing out on being in the bottom quintile!

But, as M105 demonstrates so well, it's not a high SB per se that prevents a galaxy from becoming Sunk (ignoring the main factor, SED, for now). After all, a small, high SB galaxy will quickly become indistinguishable from a star as its redshift increases (if all we do/have is rather poor broadband photometry; i.e. we cannot reliably estimate the likelihood that a point source is a galaxy and not a star or quasar, simply on its broadband colours).

So what about area? What are the most prominent Messier galaxies in terms of area? Well, we have to move them all so they're at the same distance, to get a fair comparison. When we do that, what do we find?

Stay tuned!

* i.e. taking the reported magnitudes and spreading the flux evenly over the reported area, of each galaxy. For this exercise I assumed a nice, Euclidean universe; one in which SB is independent of distance.

StupendousMan
2012-Feb-18, 01:38 AM
After all, a small, high SB galaxy will quickly become indistinguishable from a star as its redshift increases (if all we do/have is rather poor broadband photometry; i.e. we cannot reliably estimate the likelihood that a point source is a galaxy and not a star or quasar, simply on its broadband colours).


ORLY?

Please scan the figures in http://arxiv.org/abs/astro-ph/0010052 or any of a number of other papers which show how one can (usually) distinguish stars from galaxies using broad-band colors.

ngc3314
2012-Feb-18, 01:48 AM
ORLY?

Please scan the figures in http://arxiv.org/abs/astro-ph/0010052 or any of a number of other papers which show how one can (usually) distinguish stars from galaxies using broad-band colors.

Well, Nereid did specify the case where the photometry is poor (ellipticals could masquerade as K stars easily enough if close to the flux detection threshold).

Nereid
2012-Feb-21, 01:14 AM
ORLY?

Please scan the figures in http://arxiv.org/abs/astro-ph/0010052 or any of a number of other papers which show how one can (usually) distinguish stars from galaxies using broad-band colors.

Yep, and, in this case, I should have not posted at all (or not without extensive re-writing). But hindsight is always 20/20, isn't it?

In fact, I hadn't given much thought at all to this part of D&L's thesis (i.e. that small - in area - local galaxies with high SB will become indistinguishable from stars if they are moved to high z). Clearly, if I had, I'd have found something like what you posted. But then D&L's observational astronomy is conducted in a single band (by inference, I don't they state anything much about passbands, etc).

Some interesting questions: how far out can UCDs (ultra-compact dwarf galaxies) be confidently separated - by broadband photometry alson - from stars? quasars?

Anyway, time to move along.

You may be wondering what I used as a model for T12. Well, here's the answer:


Tal and van Dokkum (2011) (http://arxiv.org/abs/1102.4330): "The faint stellar halos of massive red galaxies from stacks of more than 42000 SDSS LRG images".

T12 is a composite of LRG (luminous red galaxies) at a z of ~0.34. These galaxies are (mostly?) BCGs, and are all ETGs.

Recall this post?

Here are some (edited) extracts from the preprint; I'll give full details in a later post.


Our sample of {galaxies} is drawn from the HST imaging with Advanced Camera for Surveys (ACS) and WFC3, which was obtained as part of the [...] program. Near-UV and near-IR observations were acquired as part of the WFC3 [...] program [...] a 104 orbit medium-depth survey using the HST UVIS and IR cameras. A general introduction to the performance and calibration of the WFC3 is provided in Windhorst et al. (2011). The [...] program observed approximately 50 square arcminutes in the GOODS-S field with the HST WFC3 UVIS in three filters: F225W and F275W for 2 orbits, and F336W for 1 orbit, per pointing, respectively. The program observed approximately 40 square arcminutes in the same field with the WFC3 IR in three filters: F098M, F125W, and F160W, each for 2 orbits per pointing. The 5σ 50% point-source completeness limits are: F225W=26.3, F275W=26.4, F336W=26.1, F098M=27.2, F125W=27.5, and F160W=27.2 mag (see Windhorst et al. 2011). The analysis presented here was completed using mosaicked images produced for each of the UVIS and IR band tilings, and each image mosaic was drizzled to a pixel scale equal to 0.090" pixel-1.

[...]

The WFC3 mosaics roughly cover the northern one-third of the GOODS-S field (Giavalisco et al. 2004), and we incorporate the pre-existing ACS dataset (F435W, F606W, F775W, and F850LP) with the WFC3 observations. We produced mosaicked images of the GOODS-S ACS data, which were binned to match the pixel scale of the WFC3 UVIS/IR mosaics.

Selection Criteria

We require our galaxies to have: (1) been imaged in all UV and IR bands, to uniform depth; (2) a spectroscopically-confirmed redshift in the range 0.35 ~< z < ~1.5; and (3) an {galaxy} morphology.

There are many techniques for identifying {galaxies} at intermediate redshift. [...] However, the robustness of each of these classifiers can be dramatically affected by a variety of systematics, such as the image signal-to-noise ratio (Conselice et al. 2003; Lisker 2008) and the bandpass in which the technique is applied (Taylor-Mager et al. 2007; Conselice et al. 2008). In lieu of these techniques, we select our sample by visual classification. This technique is subjective, and as such can introduce new biases, but it has been successfully applied to the identification of both low redshift (z ∼0.1; [...]) and intermediate redshift (z ~< 1.3; [...]) {galaxies}. We will demonstrate in ž{Y} that the spectroscopic redshift requirement, and not the morphological selection technique, is the most significant source of bias.

[...]

UV imaging can provide unique insight into {something interesting}. Thus, we require our sample {galaxies} to be observed in each of the UV filter mosaics. To ensure that all galaxies were observed to a similar depth, we also require each {galaxy} in the sample to be observed in the UV and IR image mosaics for at least the mean exposure time measured for each filter as given by Windhorst et al. (2011). Since we are interested in {something interesting}, and the WFC3 UVIS channel is only sensitive to UV emission at ∼ 1500 ┼ for objects at redshift z ~> 0.35, we define this redshift as low-redshift cutoff of the sample. The high-redshift cutoff was selected to ensure that the visual inspection and classification of the {galaxy} Ś in the filter set outlined above ľ considers the rest-frame V-band morphology. We are sensitive to at least the UV-optical SED of every {galaxy} in our catalog.

[...]

We find 102 {galaxies} that satisfy these selection criteria.

Photometry

We measured object fluxes using SExtractor in dual-image mode (Bertin & Arnouts 1996), with the WFC3 F160W image as the detection band. For source detection, we required sources to be detected in minimally four connected pixels, each at ≥ 0.75σ above the local computed sky-background. For deblending, we adopted a contrast parameter of 10−3 with 32 sub-thresholds. Object photometry was determined with MAG AUTO parameters Kron factor equal to 2.5 and minimum radius equal to 3.5 pixels.

We adopted gains for each filter using the mean exposure time calculated for each mosaic as follows: F225W and F275W equal to 5688 sec; F336W equal to 2778 sec and F098M, F125W, and F160W equal to 5017 sec (see Windhorst et al. 2011). From Kalirai et al. (2009a,b) we assumed zeropoints for the filter set F225W, F275W, F336W, F098M, F125W, F160W equal to 24.06, 24.14, 24.64, 25.68, 26.25, 25.96 mag, respectively. We assumed zeropoints for the filter set F435W, F606W, F775W, and F850LP equal to 25.673, 26.486, 25.654, and 24.862 mag, respectively.

In Table {X} we present the measured photometry for the {galaxies}. SExtractor non-detections are designated " Ě Ě Ě " (23 galaxies) and {galaxy} fluxes with detections fainter than the recovery limits (discussed below) are designated "Ś" (52 galaxies), as explained in the footnotes of Table {X}.

The combination of the stable WFC3 UV-optical-IR PSF and high spatial resolution allows many compact or low surface brightness (SB) {galaxy} candidates to be detected and measured. These candidates may meet the morphological selection criteria in the "detection" image, but in dual-image mode SExtractor returns flux measurements for these {galaxies} which are significantly below the formal completeness limits in the "measurement" image. Their formal flux uncertainties are larger than ∼1 mag (implying a signal-to-noise ratio ~< 1). To ascertain the reliability of these faint flux measurements in the UV bandpasses, we inserted simulated galaxies into the images, and performed an object recovery test to measure the flux level where the signal-to-noise typically approaches ∼1. To derive 90% confidence limits, we inserted ∼60,000 simulated galaxy images representing a range of total magnitudes (24 mag < m < 30 mag) and half-light radii (0.8" < rhl < 2.25") into each of the UVIS mosaics, and measured the fraction of simulated galaxies which were recovered by SExtractor, using the same SExtractor configuration as discussed above. The simulated galaxies were defined with an r1/4 ("bulge") or exponential SB profile (ü"disk"). From these simulations, we estimated the 90% recovery limits for simulated [...] profiles with half-light radius of 1.0" equal to F225W=26.5, F275W=26.6, F336W=26.4, and F435W=26.7 mag, respectively. We interpret {galaxies} with magnitudes fainter than these recovery limits as 1-σ upper limits.

Let's remind ourselves of one of Disney & Lang's most eye-catching lines: "If now we remove the Local population to redshift 1, virtually all the previously prominent galaxies will sink below the sky thanks to Eqn. (43)."

Hmm.

You'll be able to see for yourself, later, from Table {X}, just how many of the 102 galaxies examined - in considerable detail - have a redshift of >=1. Too, once I tell you what {galaxies} this (as yet unnamed) paper sought to study, you can decide if any are prominent in the "Local population" (Disney & Lang - deliberately? - do not provide a definition of what they mean by this term).

In any case, I would guess that Disney & Lang would be rather astonished by this paper; in quite a few ways it would seem - at face value - to contradict several of the main conclusions of their own. And, as in all science, experiment (or observation) always trumps theory ...

Well, the paper in question is Rutkowski et al. (http://arxiv.org/abs/1201.6416) "A Panchromatic Catalog of Early-Type Galaxies at Intermediate Redshift in the Hubble Space Telescope Wide Field Camera 3 Early Release Science Field".

Yep, once again the galaxies under study are ETGs.

But what's perhaps the most remarkable, even ironic, thing about this paper?

Stay tuned?

Nereid
2012-Feb-22, 01:29 AM
But what's perhaps the most remarkable, even ironic, thing about this paper?

Stay tuned?

No one?

Well, think about this: here is some text from the abstract of the Disney&Lang paper:


Conversely the ancestors of the Milky Way and its obvious neighbours will have completely sunk below the sky at z > 1:2,
[...]

This Succeeding Prominent Dynasties Hypothesis (SPDH) fits the existing observations both naturally and well, including the bizarre distributions of galaxy surface brightness found in deep fields, the angular size  (1+z)-1 law, 'downsizing' which turns out to be an 'illusion' in the sense that it is does not imply evolution, 'Infant Mortality', i.e. the discrepancy between stars born and stars seen, and fi nally the recently discovered and unexpected excess of QSOAL DLAs at high redshift. If the SPDH is true then a large proportion of galaxies remain sunk from sight, probably at all redshifts. We show that fi shing them out of the sky by their optical emissions alone will be practically impossible, even when they are nearby. More ingenious methods will be needed to detect them. It follows that disentangling galaxy evolution through studying ever higher redshift galaxies may be a forlorn hope because one will be comparing young oranges with old apples, not ancestors with their true descendants.

Sure that's pretty general, but how reasonable is it, in light of Rutkowski et al.'s paper? And, if Disney and/or Lang were to read Rutkowski et al.'s paper, which parts of the above - or their own paper - would they wish to at least seriously re-write?

Or, bluntly, would Rutkowski et al.'s paper come as a big surprise to Disney and Lang?

Nereid
2012-Feb-23, 01:10 AM
Or, bluntly, would Rutkowski et al.'s paper come as a big surprise to Disney and Lang?

No one willing to hazard a guess, eh?

Well, consider who the "et al." are: S. H. Cohen, S. Kaviraj, R. W. O'Connell, N. P. Hathi, R. A. Windhorst, R. E. Ryan Jr., R. M. Crockett, H. Yan, R. A. Kimble, J. Silk, P.J. McCarthy, A. Koekemoer, B. Balick, H. E. Bond, D. Calzetti, M. J. Disney, M. A. Dopita, J. A. Frogel, D. N. B. Hall, J. A. Holtzman, F. Paresce, A. Saha, J. T. Trauger, A. R. Walker, B. C. Whitmore, and E. T. Young.

There's some interesting names there, eh? Joe Silk, for one; oh, and look, M. J. Disney is an author too! :eek: :p

Could you have guessed that?

Nereid
2012-Feb-23, 01:37 AM
A cool astro-ph (http://arxiv.org/abs/1202.4328), just two days' old: "Ultra deep sub-kpc view of nearby massive compact galaxies", by Trujillo et al.:


Using Gemini North telescope ultra deep and high resolution (sub-kpc) K-band adaptive optics imaging of a sample of 4 nearby (z~0.15) massive (~10^{11}M_{sun}) compact (R<1.5 kpc) galaxies, we have explored the structural properties of these rare objects with an unprecedented detail. Our surface brightness profiles expand over 12 magnitudes in range, allowing us to explore the presence of any faint extended envelope on these objects down to stellar mass densities ~10^{6} M_{sun}/kpc^{2} at radial distances of ~15 kpc. We find no evidence for any extended faint tail altering the compactness of these galaxies. Our objects are elongated, resembling visually S0 galaxies and have a central stellar mass density well above the stellar mass densities of objects with similar stellar mass but normal size in the present universe. If these massive compact objects will eventually transform into normal size galaxies, the processes driving this size growth will have to migrate around 2-3x10^{10}M_{sun} stellar mass from their inner (R<1.7 kpc) region towards their outskirts. Nearby massive compact galaxies share with high-z compact massive galaxies not only their stellar mass, size and velocity dispersion but also the shape of their profiles and age of their stellar populations. This makes these singular galaxies unique laboratories to explore the early stages of the formation of massive galaxies.

"Our surface brightness profiles expand over 12 magnitudes in range ..." - I thought Disney&Lang showed this was simply not possible; did I miss something?

StupendousMan
2012-Feb-23, 02:44 AM
"Our surface brightness profiles expand over 12 magnitudes in range ..." - I thought Disney&Lang showed this was simply not possible; did I miss something?

Well, the reason that this new paper by Trujillo et al. can claim such a large range in surface brightness profiles is simply that the angular resolution of the images is high. As you'll see if you examine their figure 2, much of this 12-magnitude range comes from measurements within a radius of 1 arcsecond. Remember, surface brightness measures "magnitudes per square arcsecond". If you can define a very small patch of sky, with an area of very few square arcseconds, then you can compute a very large surface brightness.

Disney and Lang claimed that this wasn't possible? Could you please provide a guide to the location of such a statement in their paper?

Nereid
2012-Feb-23, 04:32 PM
Well, the reason that this new paper by Trujillo et al. can claim such a large range in surface brightness profiles is simply that the angular resolution of the images is high. As you'll see if you examine their figure 2, much of this 12-magnitude range comes from measurements within a radius of 1 arcsecond. Remember, surface brightness measures "magnitudes per square arcsecond". If you can define a very small patch of sky, with an area of very few square arcseconds, then you can compute a very large surface brightness.

Disney and Lang claimed that this wasn't possible? Could you please provide a guide to the location of such a statement in their paper?

D&L's paper is based on a toy universe in which the (azimuthly averaged?) radial SB profile of all galaxies is wavelength/band independent (crudely, a galaxy which is an exponential in the V band, will be an exponential in every band, even bands to the blue of the Lyman limit), and which is populated by only two kinds of galaxies - those with an exponential SB profile (i.e. SÚrsic index 1) and giants with a de Vaucouleurs profile ('Giant Ellipticals', i.e. SÚrsic index 4).

The former, per Fig. 3, cannot have an SB contrast of 12 mags; the latter, per Fig. 7, certainly can.

Exponential galaxies are, nearly always (?), disks; their Hubble type ranges from S0 (lenticulars) to Sm (borderline irregulars); their bulges - if they have them - may have a de Vaucouleurs profile (with an effective radius much smaller than that of the disk), or an exponential profile (and are called 'pseudo-bulges?).

The four galaxies covered in the Trujillo et al. paper have, per Table 2, SÚrsic indices ranging from 2.18▒0.24 to 3.55▒0.39; the authors refer to their apparent morphology as 'disky'*. And we know, from the excellent Lackner and Gunn paper that I briefly mentioned earlier (http://www.bautforum.com/showthread.php/120972-Bias-effects-in-galaxy-detection?p=1979023#post1979023), that modelling real galaxies with SÚrsic profiles requires a very significant proportion of them to have indices significantly different from 1 or 4.

So, it's not so much that D&L showed that an SB contrast of 12 mags is not possible as that they did not consider anything like the full range of SÚrsic profiles (local) galaxies are known to closely approximate.

On angular resolution: D&L's paper is essentially blind to this; their logic applies equally well to a local galaxy examined at typical 'ground' resolution as to a more distant one examined at 'space' resolution. The exception they explicitly cite is "The maximum heights of the two curves [...] assume a sample for which [...], typical of all Exponential galaxies, save those hundreds of pixels across", to quote one version (from Section IV). In fact, much of Section IV is about just how truly trapped we are in "our lighted cell", in terms of angular resolution (and several other factors). Of course, this section is also (mostly) about exponential galaxies only ...

* "A visual inspection of the nearby compact massive galaxies shown in Fig. 1 indicates that the most common morphology of our objects is disky. In fact, two objects (SDSS J103050.53+625859.8 and SDSS J120032.46+032554.1) visually resemble S0 galaxies viewed in edge-on projection. The other two galaxies (SDSS J153934.07+441752.2 and SDSS J212052.74+110713.1) have a more distorted morphology but still are compatible with being S0 galaxies with a lower inclination (see also Valentinuzzi et al. 2010)."

Nereid
2012-Apr-17, 02:25 PM
Here's an interesting paper: Disc scalelengths out to redshift 5.8 (http://arxiv.org/abs/1204.2263):


We compute the exponential disc scalelength for 686 disc galaxies with spectroscopic redshifts out to redshift 5.8 based on Hubble Space Telescope archival data. We compare the results with our previous measurements based on 30000 nearby galaxies from the Sloan Digital Sky Survey. Our results confirm the presence of a dominating exponential component in galaxies out to this redshift. At the highest redshifts, the disc scalelength for the brightest galaxies with absolute magnitude between -24 and -22 is up to a factor 8 smaller compared to that in the local Universe. This observed scalelength decrease is significantly greater than the value predicted by a cosmological picture in which baryonic disc scalelength scales with the virial radius of the dark matter halo.

It didn't make the Fun Papers In Arxiv (http://www.bautforum.com/showthread.php/123740-Fun-Papers-In-Arxiv/page6) cut.

I wonder what Disney and Lang thought of it (assuming they read it)? And did Fathi et al. include the Tolman surface brightness effect in their calculations, I wonder.