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rtomes
2008-Mar-18, 06:38 AM
I know that quasar stands for quasi-stellar objects so that they have a small apparent diameters, but cannot find a source giving typical apparent diameters. Can someone please tell me either a good source for this or perhaps a few typical cases such as some low redshift ones like 3C273 (I managed to find a photo but without a scale) and some higher redshift ones also if they can be resolved.

Thanks
Ray

astrotech
2008-Mar-18, 07:59 AM
Quasar means Quasi stellar. Or sort of star like. To speak in terms of apparent diameter is not correct for stars. We speak in terms of apparent magnitude or brightness. We do this because we cannot reslove the diameter of stars only their magnitude or brightness. We have to question whether the brightness as it appears is due to it's actual brightness, its nearness to us or its size.

Our sun is about 4 light seconds across. Our galaxy is a hundred thousand light years across. At the distance of quasars we can resolve galaxies well enough to see that they are galaxies. A quasar emits as much energy as an entire qalaxy. But we cannot resolve quasars like we cannot resolve stars at that distance.

So, it emits as much energy as a qalaxy but cannot be resolved like a star at that distance cannot.

So like a star in that it cannot be resolved but like a galaxy in that it emits a lot.

So it's somthing like a star, a quasi star, a quasar.

Jeff Root
2008-Mar-18, 09:15 AM
astrotech,

Your reply seems to dismiss the importance of the apparent diameters
of quasars, but knowing the apparent diameter is absolutely vital to
understanding what they are. I am as interested in knowing the apparent
diameter as Ray, because shortly after they were discovered, I thought
quasars might be the cores of very early galaxies in which supernovae
were occurring at an enormous rate -- several a day. I was told that
quasars are too small for that to be possible, but I've never seen actual
measurements of their sizes.

-- Jeff, in Minneapolis

Ari Jokimaki
2008-Mar-18, 09:56 AM
SDSS measures QSO diameters. Here's NED page for one SDSS quasar (http://nedwww.ipac.caltech.edu/cgi-bin/nph-objsearch?search_type=Obj_id&objid=7553806&objname=2&img_stamp=YES) and here's NED's page for individual diameter measurements for that object (http://nedwww.ipac.caltech.edu/cgi-bin/nph-datasearch?search_type=Diam_id&objid=7553806&objname=SDSS%20J002814.79%2B150605.1).

John Mendenhall
2008-Mar-18, 12:38 PM
Here's NED page for one SDSS quasar



Excellent reference.

Armchair astronomer folks (like me), note how much work has gone into the sites referenced by Jokomaki. This is not a casual enterprise.

George
2008-Mar-18, 01:13 PM
Why is their apparent size so small? If space were static, then I would understand them appearing small, but since space is modeled as expanding, wouldn't their apparent size be the size we would have seen them at the time the light we are now recieving actually left their galaxy?

antoniseb
2008-Mar-18, 02:23 PM
I know that quasar stands for quasi-stellar objects so that they have a small apparent diameters, but cannot find a source giving typical apparent diameters. ...

The optical light from quasars is not coming from a small central point.
I think the best measurement of the size and shape of a particular quasar did NOT come from a nearby one like 3c273, but came from study of the Einstein Cross. The study of the microlensing events in the five paths through the intervening galaxy has given information that can be seen as a map of the light source.

I don't have a pointer to this, but you should be able to find it.

StupendousMan
2008-Mar-18, 03:27 PM
Most of the visible light from a quasar is emitted by gas in an accretion disk around the central black hole. There are several ways to estimate the sizes of these accretion disks -- simple blackbody emission calculations, more detailed radiative transfer models, gravitational microlensing, spectral modeling. You can find papers describing details by going to the ADS

http://adsabs.harvard.edu/abstract_service.html

and typing into the "Abstract Words" box terms such as

quasar accretion disk radius

and pressing the "Send Query" button. You'll receive a list of many papers; read through the abstracts and pick the ones you find most promising.

For example,

http://adsabs.harvard.edu/abs/2005AJ....129.1225S
http://adsabs.harvard.edu/abs/2002MNRAS.331.1041W
http://adsabs.harvard.edu/abs/2008ApJ...673...34P

These papers provide evidence for accretion disks which range from 10^15 to 10^17 cm in radius -- or, rather, for gas at those radii which emits visible and near-IR radiation.

The apparent angular size of a "quasar" will depend on both the physical size of the accretion disk and on its distance from us, of course.

Spaceman Spiff
2008-Mar-18, 03:29 PM
A lot of confusion here (above George's post)

First, many hundreds of stars other than our Sun ARE spatially resolved - through interferometry, in eclipsing binary systems, and via other methods.

Second, the diameters (in units of arc seconds) quoted for that quasar must be of that of the host galaxy, as defined by some surface brightness criterion. At the cosmological redshift of this particular quasar, the scale is something like 40,000 pc (130,000 light years) per arc second.

Excepting the (infrequent) presence of the powerful radio jets that can be as large as 100s or 1000s of kiloparsecs, the regions that generate the luminous power of quasars (which lie at the centers of massive galaxies) are tiny, tiny, tiny compared to galactic size scales. The region that generates the continuum light from the UV to the visible in luminous quasars is no more than a ~ few tenths of a light year across (and most of that light comes from a region maybe 10x smaller), the main X-ray emitting region is likely even smaller still, while that of the emission line region is typically no more than a few light years across. Thus at the distance of this object, these light emitting regions of the quasar range from something like ~ 0.05-50 micro-arcseconds.

These size estimates come from variability arguments: the X-rays, UV and visible light continuum emission all vary in intensity, and the broad emission lines (that are powered by the UV-X-ray light) also vary but delayed in time due to the finite speed of light. As mentioned above we occasionally get spatial information of the light emitting regions from gravitational microlensing observations. We also have basic working models for these light emitting mechanisms, and although the details still elude us (keeps me and others busy in research), the observations and model predictions are in reasonable agreement.

There are plans to begin using interferometric methods to resolve the broad emission line regions (the gas responsible for the strong, very broad emission lines) in some nearby sources.

George is correct in that for the standard cosmological parameters, the angular diameter distance (aka the emission distance) reaches a peak near a redshift of 1.6 and then declines thereafter, but that's not a big effect for this redshift 2 quasar.

Nereid
2008-Mar-18, 10:52 PM
One more source of confusion: Type 2 quasars (or Type II quasars).

For these AGN galaxies, the central accretion disk is obscured, by the dust torus (or whatever), but the scattered light of the disk is still enough to dominate that from the host galaxy ... the size of these types of 'quasars', on the sky, may be much greater than the angle subtended by the accretion disk.

Oh how important it is to be sure you have a clear, consistent definition of a term like 'quasar'! :doh:

rtomes
2008-Mar-19, 12:29 AM
SDSS measures QSO diameters. Here's NED page for one SDSS quasar (http://nedwww.ipac.caltech.edu/cgi-bin/nph-objsearch?search_type=Obj_id&objid=7553806&objname=2&img_stamp=YES) and here's NED's page for individual diameter measurements for that object (http://nedwww.ipac.caltech.edu/cgi-bin/nph-datasearch?search_type=Diam_id&objid=7553806&objname=SDSS%20J002814.79%2B150605.1).
This gives four measurements of the one object as having a radius of 1.80, 0.57, 0.04 and 0.06 seconds of arc. Not exactly great consistency!!

If there are photos of 3C273 showing great details:
http://www.spacetoday.org/images/DeepSpace/Quasars/Quasar3C273Hubble.jpg
then it must be easy enough to measure, but that photo does not come with a scale. My science teacher in school would take off a lot of marks if you didn't show the scale. :-)

Anyway, an interesting photo.


One more source of confusion: Type 2 quasars (or Type II quasars).

For these AGN galaxies, the central accretion disk is obscured, by the dust torus (or whatever), but the scattered light of the disk is still enough to dominate that from the host galaxy ... the size of these types of 'quasars', on the sky, may be much greater than the angle subtended by the accretion disk.

Oh how important it is to be sure you have a clear, consistent definition of a term like 'quasar'! :doh:

Well if there are differences for different types of quasars, I would like to know about that. But what I am really after is the typical optical diameter of quasars, perhaps as a function of redshift. If they often have a black spot on them like 3C273 then it would be good to know about the typical size of that too (as a function of redshift).


Most of the visible light from a quasar is emitted by gas in an accretion disk around the central black hole. There are several ways to estimate the sizes of these accretion disks -- simple blackbody emission calculations, more detailed radiative transfer models, gravitational microlensing, spectral modeling. You can find papers describing details by going to the ADS

http://adsabs.harvard.edu/abstract_service.html

and typing into the "Abstract Words" box terms such as

quasar accretion disk radius

and pressing the "Send Query" button. You'll receive a list of many papers; read through the abstracts and pick the ones you find most promising.

For example,

http://adsabs.harvard.edu/abs/2005AJ....129.1225S
http://adsabs.harvard.edu/abs/2002MNRAS.331.1041W
http://adsabs.harvard.edu/abs/2008ApJ...673...34P

These papers provide evidence for accretion disks which range from 10^15 to 10^17 cm in radius -- or, rather, for gas at those radii which emits visible and near-IR radiation.

The apparent angular size of a "quasar" will depend on both the physical size of the accretion disk and on its distance from us, of course.
Not only useful information but also "how to do it" for future reference, thanks StupendousMan.

However the first one of these papers abstract starts:

Because quasars are unresolved in optical imaging, their structures must currently be inferred.
Why would they say that when there are photos of quasars that show details, and various reports about movements in parts of quasars (some that appear faster than c) and other such stuff published regularly?

Thanks to all those that supplied information.

Spaceman Spiff
2008-Mar-19, 01:45 AM
rtomes -

That image is the image of the host galaxy of 3C 273. It is enormous in size (like Milky Way), in comparison to the quasar (whose light was blotted out in the image you posted). The "inconsistencies" in sizes noted are not such, but the different results using different measures of different size scales.

Spaceman Spiff
2008-Mar-19, 01:47 AM
One more source of confusion: Type 2 quasars (or Type II quasars).

For these AGN galaxies, the central accretion disk is obscured, by the dust torus (or whatever), but the scattered light of the disk is still enough to dominate that from the host galaxy ... the size of these types of 'quasars', on the sky, may be much greater than the angle subtended by the accretion disk.

Oh how important it is to be sure you have a clear, consistent definition of a term like 'quasar'! :doh:

Yeah, but even then the obscuring 'torus' isn't really considered part of the quasar, or if one were to include it (as the most immediate supply of gas to the inner accretion disk surrounding the supermassive black hole), then it's also present in all/most quasars (and it only gets in the way when we view the system edge-on).

Nereid
2008-Mar-19, 02:31 AM
And I think that's part of the problem ... if the term is used to mean just the accretion disk (more or less), then indeed it will be tiny (in arcseconds).

However, I think you'll find that the word* is used with a range of different meanings, not only in different contexts, by different authors, and so on, but also it's changed in the last four decades or so.

I think rtomes, in the OP, was referring to an observational definition, as used by astronomers working in the visual waveband, and (likely) before the unified AGN model was much more than a topic of morning tea time speculation.

If that's so, then 'quasar' could mean the host galaxy (+torus+jets+...) and, IIRC, even some 40 years ago there were papers discussing the ~2" or so 'fuzz' that seemed to surround some of the newly discovered quasars.

* Along with QSO, AGN, and, no doubt, many others

neilzero
2008-Mar-19, 04:05 AM
Of course we are seeing quasars as the looked billions of years ago. Likely they are much dimmer now, and perhaps even smaller, as the reduced photon and ion pressure would allow the orbits to contract. Generally galaxtic groups and smaller things do not universe expand with the space between the galaxtic groups. There should be a former quasar within a few million light years of Earth, perhaps much closer. Any other ideas what to look for? Do typical quasars send us light that was red shifted mostly from gamma when it left the quasar? Neil

Nereid
2008-Mar-19, 08:03 AM
Re post#11:

That's an image of 3C 273, taken by the Hubble Space Telescope's Advanced Camera for Surveys, using the camera's coronograph*, and part of News Release Number: STScI-2003-03 (http://hubblesite.org/newscenter/archive/releases/2003/03/fastfacts/) (note that the scale is given).

You need to read the details of how the relevant SDSS data reduction pipeline(s) makes estimates of 'diameters' to understand what the differences are in the r band major axis estimates of SDSS J002814.79+150605.1, though the "Detailed Information for Each Entry" gives you some clues.

* That's what produces the 'black spot'!

mugaliens
2008-Mar-19, 05:14 PM
Excellent reference.

Armchair astronomer folks (like me), note how much work has gone into the sites referenced by Jokomaki. This is not a casual enterprise.

You're absolutely right. It's getting deep into the heart of astrophysics.

So, I third your opinion.

mugaliens
2008-Mar-19, 05:33 PM
Why would they say that when there are photos of quasars that show details, and various reports about movements in parts of quasars (some that appear faster than c) and other such stuff published regularly?

Thanks to all those that supplied information.

I believe you're absolutely right in requiring some sort of a published scale.

How about (and I'm only a very amateur astronomer) ((so please feel freel to add)):

All Internet-posted astronomical objects are required to include means of telemetry (details on specific devices used during the observation), observation data (inclination, degrees, date/time of observation, nomenclature of object being observed (if applicable)).

Cut to the chase, gentleman, just putting out "ooh/aah" shots of glorious clusters is no longer acceptable.

Give us the specific information along with the shots.

Is that such a difficult task, paticularly given the fact that my 7th grade astronomy teacher has most of that programmed into his computer-controlled telescope, but can't access the details (like where it is in space?) when he reads it online if what I consider the most prominent Astronomy forum available, today?

Uh, duh, we're a bit brighter than that these days (I would think), and if you're having a database problem, please bring it on.

I can fix it.

I've solved more than a few, and have, over thirty years, discovered that most such problems are because most people don't understand data.

I do. That's what I do.

parejkoj
2008-Mar-19, 06:13 PM
This gives four measurements of the one object as having a radius of 1.80, 0.57, 0.04 and 0.06 seconds of arc. Not exactly great consistency!!


Those different sizes are all consistent with the object being a point source. They are also very different "sizes:" deVaucouleurs and Exponential are galaxy fits (and the radius is the half-light radius), a description of the Petrosian magnitude is on the SDSS website (http://cas.sdss.org/dr6/en/help/docs/algorithm.asp?key=mag_petro), and the isophotal radius is the semi-major axis of an elliptical gaussian fit. Only the isophotal radius really means anything in this case, since the Sloan PSF is ~1.4, so sizes smaller than that are meaningless (and in this case, they mean that those models aren't good fits). The SDSS pipeline fits all models to every source, which is why they are all reported on NED.



If there are photos of 3C273 showing great details:
then it must be easy enough to measure, but that photo does not come with a scale. My science teacher in school would take off a lot of marks if you didn't show the scale. :-)


It's not at all easy to measure. The quasar is much brighter than the host galaxy, so you either need a very solid understanding of the PSF (as a group did using adaptive optics on the gemini telescope (http://www.journals.uchicago.edu/doi/abs/10.1086/381507)) or some way to block out the central point source, as was done for the Hubble ACS image.

But where the heck did you find just the picture? Nereid's link to the press release has all the details, including a scale, so I don't know how you would have missed them. Here's the paper describing the research that went into that image (http://www.iop.org/EJ/article/1538-3881/125/6/2964/202549.text.html). Took me about 30 seconds to find (google search for 3c273 hubble ACS)...



Well if there are differences for different types of quasars, I would like to know about that.


I've given you plenty of links in other threads about this. There are also some relevant links in my What's a quasar (http://www.bautforum.com/astronomy/64730-what-dickens-quasar.html) thread, including a history of AGN.



But what I am really after is the typical optical diameter of quasars, perhaps as a function of redshift. If they often have a black spot on them like 3C273 then it would be good to know about the typical size of that too (as a function of redshift).


Though StupendousMan and Spaceman Spiff have all pretty much said this above, I'll just reiterate:
All quasars are point sources to all current optical detectors (though, I think a few of the nearest bright AGN might have their broad-line region resolvable whenever the VLT interferometer is working at maximum spec). The "black spot" is the coronograph on ACS, which blocked out the quasar's light to show the host galaxy. Even the so-called "narrow line region" is of order a hundred light years across, which is too small to image. The broad line region is maybe a couple light years and the accretion disk a few hundred AU, at most. That's pretty small.

If you really want more information, why don't you read some of the links that I've posted in other threads of yours (like the one you have going in ATM right now), hmmm?



There should be a former quasar within a few million light years of Earth, perhaps much closer. Any other ideas what to look for?


Neil: we've got a ~4 million solar mass black hole in our own galaxy: SgrA*. There are two main groups that study it; UCLA, headed by Prof. Andrea Ghez (http://www.astro.ucla.edu/~ghezgroup/gc/) and Max-Planck headed by Prof. Reinhard Genzel (http://www.mpe.mpg.de/ir/GC/index.php). However, SgrA* was probably never a quasar: it isn't big enough, and our galaxy hasn't had enough big interactions. M87 (http://csep10.phys.utk.edu/astr162/lect/active/smblack.html) is probably the best candidate for a nearby former quasar, though M84 (described at the same link) is another good one.

And I'll have to respectfully disagree with Spaceman Spiff in this case: the term quasar often refers to the whole package, including the accretion disk, disk winds and corona, broad line region, narrow line region, dusty/clumpy/bumpy torus, etc. But typically not the host galaxy (which is called exactly that), and usually not the jet (unless the jet is point right at us, in which case we have a Blazar...). That's not to say that it should refer to all of that, instead of just the accretion disk, but those features are all fairly common to accreting supermassive black holes.

John Mendenhall
2008-Mar-19, 07:12 PM
Cut to the chase, gentleman, just putting out "ooh/aah" shots of glorious clusters is no longer acceptable.



My favorites are the pictures of starfields with no coordinates, no arrows, no distiguishing marks, no references, and a caption that reads something like "Notice the interesting red dwarf" (or brown dwarf, or faint object, or whatever). Maddening.

ngc3314
2008-Mar-19, 09:56 PM
My favorites are the pictures of starfields with no coordinates, no arrows, no distiguishing marks, no references, and a caption that reads something like "Notice the interesting red dwarf" (or brown dwarf, or faint object, or whatever). Maddening.

But how do you enforce penalties on the myriad sites that just repost images? (Although I do admit to knowing of cases where this was done to keep competitors off the trail before publication, and at least once to help enforce a journal's prepublication press embargo - something increasingly anachronistic these days)

Dave Hogg at NYU as been working on how to undo this problem - the goal of astrometry.net is to be able to find the coordinate system and orientation of any astronomical image (and if it has enough stars, the approximate epoch as well using proper motions).
"We have built this astrometry service to create correct, standards-compliant astrometric meta-data for every useful astronomical image ever taken, past and future, in any state of archival disarray. We hope this will help organize, annotate and make searchable all the world's astronomical information." (http://www.astrometry.net) They asked early on, and I certainly contributed some sample data in what I thought was an impressive state of archival disarray. It's now in alpha test; there is a version of the code and databases available to download for those who want to experiment wholesale. I hope their definition of "standards-compliant" is less expansive than a lot of FITS coordinate systems I'e seen.

And as much as I've been sitting on my hands for the rest of this thread - yes, as far as I am aware, no measurement at optical wavelengths has ever clearly distinguished the continuum spike (or broad-emission line region) of a quasar or type 1 Seyfert galaxy from the instrumental point-spread function. This applies to Hubble imaging, where the difficulty in subtracting the point source to see detail in host galaxies lies in how minute focus changes with spacecraft temperature ("breathing" ) change the details of the diffraction pattern between quasar and reference-star observations, and look indistinguishable from trying it on one star based on the diffraction pattern of another.

Spaceman Spiff
2008-Mar-19, 10:41 PM
And I'll have to respectfully disagree with Spaceman Spiff in this case: the term quasar often refers to the whole package, including the accretion disk, disk winds and corona, broad line region, narrow line region, dusty/clumpy/bumpy torus, etc. But typically not the host galaxy (which is called exactly that), and usually not the jet (unless the jet is point right at us, in which case we have a Blazar...). That's not to say that it should refer to all of that, instead of just the accretion disk, but those features are all fairly common to accreting supermassive black holes.

Thanks for all of your additional info.

I'll have to respectfully disagree with your disagreement...but not entirely....:dance:

We seem to be in agreement that that size scale of the quasar phenomenon is a fuzzy issue. The Narrow Line Region stretches out to scales that are normally considered "galactic" rather than "nuclear" environment, even in the lower luminosity "Seyfert" type of active galactic nuclei. Quasars also photoionize gas and produce Lyman alpha halos around their host galaxies on scales of ~100 kpc. The powerful relativistic particle jets (in a minority of quasars) extend even further beyond.

So I will suggest that when the question of "resolving" a quasar comes up, one should be careful to frame the question around the specific phenomenon related to the quasar activity.

Of course, the host galaxy itself would not be tagged as part of the quasar phenomenon.

parejkoj
2008-Mar-19, 10:52 PM
I'll have to respectfully disagree with your disagreement...but not entirely....:dance:


Are we agreeing that we disagree about our disagreement? Or are we disagreeing about that too? :confused:



We seem to be in agreement that that size scale of the quasar phenomenon is a fuzzy issue. The Narrow Line Region stretches out to scales that are normally considered "galactic" rather than "nuclear" environment, even in the lower luminosity "Seyfert" type of active galactic nuclei.


No disagreement there. :whistle:

Well, except in what you term (or not) the "nuclear" environment... I'd call within ~100pc or so, nuclear. But now I think we're just quibbling over details...



So I will suggest that when the question of "resolving" a quasar comes up, one should be careful to frame the question around the specific phenomenon related to the quasar activity.


Certainly.

Oh, and which quasar is that in your profile image?

Nereid
2008-Mar-19, 11:37 PM
3c 273

Spaceman Spiff
2008-Mar-20, 01:50 PM
Yep.

Spaceman Spiff
2008-Mar-20, 01:52 PM
Are we agreeing that we disagree about our disagreement? Or are we disagreeing about that too? :confused:

yes. :dance:

grav
2008-Mar-21, 08:44 PM
Why is their apparent size so small? If space were static, then I would understand them appearing small, but since space is modeled as expanding, wouldn't their apparent size be the size we would have seen them at the time the light we are now recieving actually left their galaxy?I'm thinking that since space is modelled as expanding, then it is not so much the relative speed between distant galaxies and an observer that causes a redshift, but that distant galaxies can be regarded as static in their own right, and it is space itself that expands, dragging them along with it somehow, but otherwise the galaxies remain stationary. This being the case, then in this sense it is the wavelength of the light itself, or the distance between wavecrests or individual photons, which would expand along with the expansion of the universe that causes a redshift over time, or over a distance travelled. But while the light is in transit, the expansion should also bring the observer further and further away from the wavefront of the light at the same time, I think, so the light ends up having further to travel. The lines of sight should also increase accordingly over the greater distance travelled, and by the time it reaches the observer, the angles for the lines of sight would probably just about match that for the current distance of the galaxy, not for the distance that it was when the light was originally emitted.

George
2008-Mar-21, 10:27 PM
I'm thinking that since space is modelled as expanding, then it is not so much the relative speed between distant galaxies and an observer that causes a redshift, but that distant galaxies can be regarded as static in their own right, and it is space itself that expands, dragging them along with it somehow, but otherwise the galaxies remain stationary. This being the case, then in this sense it is the wavelength of the light itself, or the distance between wavecrests or individual photons, which would expand along with the expansion of the universe that causes a redshift over time, or over a distance travelled. But while the light is in transit, the expansion should also bring the observer further and further away from the wavefront of the light at the same time, I think, so the light ends up having further to travel. Isn't that how Doppler works, too? I have no problem with the view that galaxies have the gravitational strength necessary to easily overcome the expansion force that is acting on spacetime itself, and in agreement with the first part of your statement. But, as our galaxy expands away from the others, will it not gain relative velocity such that when the final photon packet arrives we will be traveling so much faster than when the photons were emitted that redshift must be observed? Don't take my view as carrying much weight, as this stuff is generally over my head.

Yet, I don't think anyone has guessed the result of one of my little gedankenexperiments. Place a mirror on a long pole, say 1 billion light years long, then shine a monochromatic laser of your choice, I like green, at the mirror and wait to see what color it is when it returns. If it returns green, then space does not streetch the photons, otherwise, it probably does. I don't know if that even qualifies as a gedankenexperiment, but it illustrates a distinction that should exist, I think.


The lines of sight should also increase accordingly over the greater distance travelled, and by the time it reaches the observer, the angles for the lines of sight would probably just about match that for the current distance of the galaxy, not for the distance that it was when the light was originally emitted. I suspect you are correct, and that Spaceman Spiff's earlier answer is right, too. I would bet the expansion rate determines the changes to apparent size, but has little effect for great distances, as you said.

grav
2008-Mar-22, 01:06 AM
Isn't that how Doppler works, too?As far as the mathematics go, we can equally apply an expanding of space that increases wavelengths or a relative speed between galaxies that move apart with space due to the expansion, so it should probably work out the same using relativistic Doppler, yes.



Yet, I don't think anyone has guessed the result of one of my little gedankenexperiments. Place a mirror on a long pole, say 1 billion light years long, then shine a monochromatic laser of your choice, I like green, at the mirror and wait to see what color it is when it returns. If it returns green, then space does not streetch the photons, otherwise, it probably does. I don't know if that even qualifies as a gedankenexperiment, but it illustrates a distinction that should exist, I think.Interesting experiment. On the one hand, the mirror remains stationary with the observer since expansion can't overcome the internal forces of the pole, so we should see no redshift. But on the other hand, the wavelengths are free to expand while the light is in transit, so should continue to redshift with the expansion of the universe. I think the redshift would still be observed in general, but the rate of expansion is so small that it is normally just neglected with relativistic Doppler. Tired light theory would also demonstrate this effect (as would "shrinking galaxies", which is synonymous with "expanding universe" in my opinion), so I think observing a redshift would be pretty much the case any way you go with it.

George
2008-Mar-22, 03:21 AM
Interesting experiment. On the one hand, the mirror remains stationary with the observer since expansion can't overcome the internal forces of the pole, so we should see no redshift. But on the other hand, the wavelengths are free to expand while the light is in transit, so should continue to redshift with the expansion of the universe. I think the redshift would still be observed in general, but the rate of expansion is so small that it is normally just neglected with relativistic Doppler. Tired light theory would also demonstrate this effect (as would "shrinking galaxies", which is synonymous with "expanding universe" in my opinion), so I think observing a redshift would be pretty much the case any way you go with it. It is one or the other. Maybe we should do a poll. [That's my lousy Friday pun, looks like you got it. :)]

I just can not imagine the expansion of space forcing its way into the strong region of the electromagnetic waves. We'll let the gravitational force subdue the expansion for galaxies, but the potent photon must yield? I say the laser leaves green, and green it stays. [where did the folding arm icon run off to? :)]

Jeff Root
2008-Mar-22, 08:23 AM
Place a mirror on a long pole, say 1 billion light years long, then shine a
monochromatic laser of your choice, I like green, at the mirror and wait
to see what color it is when it returns. If it returns green, then space
does not streetch the photons, otherwise, it probably does.
I forgot that you suggested this thought experiment a long time ago.

We just had an intense and productive discussion of this question in
the thread
http://www.bautforum.com/questions-answers/71102-prove-me-space-expands-i-think-galaxies-move-static-space.html

Ken G provided 95% of the productive answers. I contributed by asking
the question over and over again.

I asked whether space itself is physically expanding, or whether the
distances between galaxy clusters is increasing without the space they
are in (or the space between them) "doing" anything.

Ken's answer seems to be that neither concept is correct. How we
describe what happens is the result of our choice of coordinate system.
One coordinate system describes space as if the galaxies were moving
through it, and all the redshift seen results from Doppler. That system
has the problem that it is difficult to relate measurements in different
regions. Another coordinate system, called the co-moving coordinate
system, which expands with the increasing distances between galaxy
clusters, describes space as if it is expanding physically, and is the
primary cause of the redshift, rather than Doppler shift. This system
has the advantage that measurements between different regions can
be easily related to each other. However, Ken says, neither system
is more "right" than the other. They are just mathematical descriptions
of the same observed phenomenae.

I'm not entirely satisfied with Ken's answer. I tried to suggest the
equivalent of your mirror-on-a-pole experiment, but Ken rejected the
effect as something that could not, even in principle, be detected.
I haven't yet followed up on that line of questioning.

I agree with you that if space itself is physically expanding, light
traversing the space between two points a fixed distance apart
(the ends of the pole) would be redshifted, while it would not be
redshifted if space itself is not physically expanding. Two clearly
distinguishable situations, not caused by the choice of coordinate
system.



I would bet the expansion rate determines the changes to apparent
size, but has little effect for great distances, as you said.
The excellent graphics and concise text on this page:
The Distance Scale of the Universe (http://www.atlasoftheuniverse.com/redshift.html)
address your question, but do they answer your question?

-- Jeff, in Minneapolis

Spaceman Spiff
2008-Mar-22, 03:18 PM
May I suggest going to my page of links on Cosmology and Cosmic Structure Evolution (http://homepages.wmich.edu/%7Ekorista/cosmology.html) and scroll to the bottom, where you'll see a figure showing a plot of the co-moving distance (distance "now"), the emission distance (aka, distance "then" or the angular diameter distance) and the lookback time * c as functions of redshift. Explanation follows. You can also go to this informative webpage (http://www.atlasoftheuniverse.com/redshift.html) (oops - already linked by Jeff Root just above).

George
2008-Mar-22, 04:22 PM
Ken's answer seems to be that neither concept is correct. ... However, Ken says, neither system is more "right" than the other. They are just mathematical descriptions of the same observed phenomenae. Yes. I think I even understand him. The results are the same if one elects to use a Doppler only approach. But that is a mathematical convenience, and may not work in every case, such as the pole-mirror experiment.


I'm not entirely satisfied with Ken's answer. I tried to suggest the
equivalent of your mirror-on-a-pole experiment, but Ken rejected the
effect as something that could not, even in principle, be detected.
I haven't yet followed up on that line of questioning. It was Ken's equivalence explanations that, I think, prompted the pole-mirror experiment. [I probably should go back there and re-read the remarks.]


I agree with you that if space itself is physically expanding, light
traversing the space between two points a fixed distance apart
(the ends of the pole) would be redshifted, while it would not be
redshifted if space itself is not physically expanding. This would be true if the expansion of space directly alters the wavelength of light, which I suggest it does not. If space is expanding away from Earth, then I would expect our photons to be blueshifted as it hits the mirror since they are given greater energy with the carry. On the return trip, however, the expansion will redshift the photons for the same reason. But, if the expansion rate is constant, then the net color will arrive the same as it left.

Alternatively, if expansion creeps in among the photon packet and streeetches the waves, then it will do so during both trips to and from the mirror. Thus, they would arrive redshifted.

Spaceman Spiff
2008-Mar-22, 06:37 PM
George - be careful there in the application of the Doppler Effect to the cosmological redshifting effect. If you re-read what KenG said carefully, I think you'll find that he said that it should not be considered applicable in the case of the gravitational field over great distance scales (different GR frames of reference). The equivalence he was discussing was a limited one.

But I think we're digressing....

George
2008-Mar-22, 07:54 PM
Yes, I am not very sharp on these things, and I greatly respect Ken's views. We are digressing, and I think I can add little else to the pole-mirror experiment for now, so I'll save it for a more appropriate thread.

rtomes
2008-Mar-31, 10:29 AM
Re post#11:

That's an image of 3C 273, taken by the Hubble Space Telescope's Advanced Camera for Surveys, using the camera's coronograph*, and part of News Release Number: STScI-2003-03 (http://hubblesite.org/newscenter/archive/releases/2003/03/fastfacts/) (note that the scale is given).

You need to read the details of how the relevant SDSS data reduction pipeline(s) makes estimates of 'diameters' to understand what the differences are in the r band major axis estimates of SDSS J002814.79+150605.1, though the "Detailed Information for Each Entry" gives you some clues.

* That's what produces the 'black spot'!
Thank you Nereid. The picture is clearly labeled QSO 3C273, so is Spaceman Spiff correct in saying the following of the previously posted version of that image?

rtomes -

That image is the image of the host galaxy of 3C 273. It is enormous in size (like Milky Way), in comparison to the quasar (whose light was blotted out in the image you posted). The "inconsistencies" in sizes noted are not such, but the different results using different measures of different size scales.

Well yes, it seems the central part was blotted out. Why is that? And how big was the central part?

rtomes
2008-Mar-31, 10:37 AM
...
But where the heck did you find just the picture?
...
You might well ask...

I did a google search without success, so clicked "google images" and got the picture, but couldn't find an associated article. Altogether I found about 5 copies of that image, but not the one with the scale on it. I thought I was being quite a good detective, but came up blank.

rtomes
2008-Mar-31, 10:47 AM
Thanks for all the answers. However I don't really want to know about the effect of expansion or long periods of time since the light left. I simply want to know the observed apparent diameters as we actually see them right now, preferably with some relationship to z.

It cannot be the case that quasar sizes are not measured or are to small because there are any number of articles about movements within jets of quasars that seem to be FTL and other such things. You cannot observe jets within a quasar without having a pretty accurate picture of how big it is. I am surprised that it is so difficult to find this information anywhere.

Parejkoj, I went to the thread that you mentioned and searched for "size" and "diameter" and such without finding anything like what I wanted to know.

Jeff Root
2008-Mar-31, 05:22 PM
Ray,

The observed jets are vastly larger than the quasars themselves.

The quasar is the starlike image. A visible jet can be tens of thousands of
light-years long, comparable in length to the diameter of a galaxy.

The center of the 3C273 image was blocked out so that the galaxy could
be seen. If the light of the quasar were not blocked, it would saturate that
whole area of the CCD imaging chip.

-- Jeff, in Minneapolis

parejkoj
2008-Mar-31, 05:56 PM
I simply want to know the observed apparent diameters as we actually see them right now, preferably with some relationship to z.


And we've told you: all the parts of the AGN that are directly influenced by the accretion disk (except for the jet) are too small to image directly, except in a handful of the nearest sources.



It cannot be the case that quasar sizes are not measured or are to small because there are any number of articles about movements within jets of quasars that seem to be FTL and other such things. You cannot observe jets within a quasar without having a pretty accurate picture of how big it is. I am surprised that it is so difficult to find this information anywhere.


Jets extend to distances well beyond the visible light radius of the host galaxy, which is why we can see them. They are also often visible in radio wavelengths, and can thus be imaged to very small distances using VLBI techniques, which is where we have observed apparent superluminal motion (actually due to projection effects). The jet in 3c273 extends to more than 10" beyond the central point source, which corresponds to a huge physical scale (more than a hundred kiloparsecs).

One way to get an estimate of the size of various parts of a quasar (and others above have already commented on this) is to look at how the light output varies with time. For example, if the continuum is variable on ~week timescales, then the region producing the continuum must be less than ~a lightweek in size. But this does not require actually resolving the emitting region...


Parejkoj, I went to the thread that you mentioned and searched for "size" and "diameter" and such without finding anything like what I wanted to know.

I don't know that I talked about size in that thread, just about the different types of AGN. Look for a link to a history of AGN paper, which goes into more detail.

rtomes
2008-Mar-31, 10:40 PM
Ray,

The observed jets are vastly larger than the quasars themselves.

The quasar is the starlike image. A visible jet can be tens of thousands of
light-years long, comparable in length to the diameter of a galaxy.

The center of the 3C273 image was blocked out so that the galaxy could
be seen. If the light of the quasar were not blocked, it would saturate that
whole area of the CCD imaging chip.

-- Jeff, in Minneapolis
Thank you Jeff.


And we've told you: all the parts of the AGN that are directly influenced by the accretion disk (except for the jet) are too small to image directly, except in a handful of the nearest sources.
...
Thanks parejkoj.

I got it now. I didn't appreciate that the jet was not counted as part of the quasar.

Perhaps another question that will set a limit on sizes - what is the best resolution of instruments that are used for this sort of work these days?

I see a new paper in arxiv that they are able to detect planets now by averaging many images from pixelated pictures so that the distance between stars is measured down to +/-0.003 pixels. Amazing! Of course that is distances not resolution.