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iron4
2004-May-15, 02:23 PM
Hi
I've been time wondering what's the density of galaxies in the Universe. yeah, this universe with its spongeous aspect, with great voids surrounded by superclusters. Amazing. But I'm trying to find if there has been some study over a LARGE volume of the universe, including a lot of voids and superclusters, that can give a cipher of what is the density of galaxies in the universe How many galaxies there are per cubic ly. That's my question
Regards

Normandy6644
2004-May-15, 04:05 PM
I'm pretty sure that there are several groups of astronomers who are currently working on a 3d "map" of all the galaxies in the universe. From current data it might even be possible to find this "galactic density" though I think once the other work is done it will be more accurate.

Kullat Nunu
2004-May-15, 08:23 PM
I'm pretty sure that there are several groups of astronomers who are currently working on a 3d "map" of all the galaxies in the universe. From current data it might even be possible to find this "galactic density" though I think once the other work is done it will be more accurate.

Well, I don't know if there is any all-sky galaxy distance mapping programs underway, but for example Sloan Deep Sky Survey (http://www.sdss.org) aims to map galaxies in a quarter of sky.

The 2dF Galaxy Redshift Survey (http://www.mso.anu.edu.au/2dFGRS/) mapped two thin sky slices. Even they are good indicators how the galaxies are distributed.

And, of course, whole sky galaxy surveys are practically impossible because our own Milky Way Galaxy blocks out large zone (10 % - 20 % of the sky depending on the wavelength) from our view (known as the Zone of Avoidance).


How many galaxies there are per cubic ly.

Maybe cubic megaparsec would be more useful. A light year is tiny in this scale!

Brady Yoon
2004-May-15, 08:41 PM
Light years aren't the correct scale for galaxies. :) Our own galaxy is 100,000 ly wide.

Cougar
2004-May-16, 12:46 AM
Hi
I've been time wondering what's the density of galaxies in the Universe. yeah, this universe with its spongeous aspect, with great voids surrounded by superclusters. Amazing. But I'm trying to find if there has been some study over a LARGE volume of the universe, including a lot of voids and superclusters, that can give a cipher of what is the density of galaxies in the universe How many galaxies there are per cubic ly. That's my question
Regards
Well, this space.com site (http://www.space.com/scienceastronomy/astronomy/universe_density_010307.html) says....

A team of researchers that looked at data from more than 140,000 galaxies says the universe is far from heavy and has a density of next to nothing.... "It's about 300 billion billion billion times less dense than water," said John Peacock of the University of Edinburgh, "or one ten-thousandth of an ounce in a volume the same size as the Earth."
So I'm not sure how that works out in galaxies per Mpc, but it must not be many!

Brady Yoon
2004-May-16, 05:27 AM
Wouldn't black holes make the density infinite? Infinity+a very small number=infinity.

Kaptain K
2004-May-16, 07:14 AM
Wouldn't black holes make the density infinite? Infinity+a very small number=infinity.
The density of the singularity is infinite, since its volume is zero. However, the mass of the black hole is not infinite. Therefore the density of space (outside of the singularity) containing the black hole is also not infinite.

glen chapman
2004-May-16, 10:23 PM
One thing that has always interested me is the findings that the density of galaxies in the universe is something like 10 times that found with stars in galaxies.

If starts were distributed like galaxies. We'd be almost seeing sun spots on our nearest neighbours....

Kullat Nunu
2004-May-17, 08:56 AM
One thing that has always interested me is the findings that the density of galaxies in the universe is something like 10 times that found with stars in galaxies.

Only 10 times?

For example Milky Way's distance from the Andromeda galaxy is about 260 times its diameter.
Distance between the Sun and Proxima Centauri is 40 million times the Sun's diameter.


If starts were distributed like galaxies. We'd be almost seeing sun spots on our nearest neighbours....

If Proxima Centauri were at distance of 365 million kilometers, then no doubt we'd see them ;)

ngc3314
2004-May-17, 11:04 AM
I'm pretty sure that there are several groups of astronomers who are currently working on a 3d "map" of all the galaxies in the universe. From current data it might even be possible to find this "galactic density" though I think once the other work is done it will be more accurate.

Well, I don't know if there is any all-sky galaxy distance mapping programs underway, but for example Sloan Deep Sky Survey (http://www.sdss.org) aims to map galaxies in a quarter of sky.

The 2dF Galaxy Redshift Survey (http://www.mso.anu.edu.au/2dFGRS/) mapped two thin sky slices. Even they are good indicators how the galaxies are distributed.

And, of course, whole sky galaxy surveys are practically impossible because our own Milky Way Galaxy blocks out large zone (10 % - 20 % of the sky depending on the wavelength) from our view (known as the Zone of Avoidance).


How many galaxies there are per cubic ly.

Maybe cubic megaparsec would be more useful. A light year is tiny in this scale!

There are two problems in getting such an average - first, we (think our surveys) are complete only in tiny local areas (like the Local Group). Since most galaxies are intrinsically faint dwarf systems, we become increasingly biased toward fainter galaxies at larger distances. No problem in principole, as long as the average ratio of all galaxies brighter than some limit you set (say to exclude wandering globular clusters) is known and constant on large scales. In that case we can extrapolate, more and more strongly as we become progressively incomplete, to the total from the brightest ones.

The second point is the on that started the thread - actual structure in the galaxy distribution, encompassing voids, filaments, walls, etc. The 2dF survey (already linked) made a good case for having gotten to the top of the structure, on scales of, mumble mumble, the best part of a billion light-years, so on this scale we finally have direct evidence for applying the Cosmological Principle.

Either way, the number of galaxies per cubic megaparsec is dominated by the faintest dwarf system, while the total luminosity can be based the other way.

felix
2013-Oct-24, 03:17 AM
see http://adsabs.harvard.edu/full/1946MNRAS.106..121F,
where Fletcher in 1946 concludes that there are several galaxies per cubic MPC.

Although I'm sure that measurements have changed since then, I cannot find a similar
simplistic result anywhere, although it might be possible to follow the publications
that referenced that paper since that time to find more modern such estimates.

I've done a calculation that might be sort of a backwards way to go about finding
such a number. I first took the best empirically observed density of the universe value including
the luminous and dark matter that makes the galaxies spin as they do, but not
using the 'dark energy' density. Then I came up with an average mass per galaxy,
which is a bit arbitrary and interesting calculation since there are several types of
galaxies, ....ellipticals, spirals, irregulars, etc, which exponentially vary in size and
mass , even among the same type of galaxy. I used a geometric mean average
to find an average galactic mass per galaxy among them, then I divided rho, the observed density of the universe,
by that average mass per galaxy, to give a rough value for galaxies per cubic megaparsec.

I might be able to dig out that calculation if interested. But surely there are less crude
and more advanced calculations that exist.

parkner
2013-Nov-08, 08:24 PM
We have probably about 10^10 big galaxies (similar to the Milky Way or bigger) in sphere of 13.7e9 ly ~ 10 Gly,
then the average distance between galaxies is about 10 Mly;

So, here is one galaxy per 3 (or 9 ?) cubic Mpc, maybe some more: 1 galaxy per 1 cubic Mpc.

neilzero
2013-Nov-08, 10:26 PM
Likely that makes sense to exclude galaxies with less luminosity than the Milky Way, as they are generally not detectable even one billion light years away? Could we detect an isolated galaxy at one billion light years about as bright (magnitude 30 perhaps?) as the Small Magelenistic cloud? How much less luminous can a dwarf galaxy be before it considered something else? Sorry the ancient topics are more interesting than the recent topics. Neil

parkner
2013-Nov-09, 04:40 AM
The Magellanic Clouds, and many other similar dwarfs, are the satellites only.
These are rather the parts of the Milky Way - a periphery, not real galaxies.

http://arstechnica.com/science/2013/01/half-of-andromedas-satellite-galaxies-orbit-in-a-mysterious-disk/

Shaula
2013-Nov-09, 07:37 AM
The Magellanic Clouds, and many other similar dwarfs, are the satellites only.
These are rather the parts of the Milky Way - a periphery, not real galaxies.
The LMC has a prominent barred-spiral-like bar indicating it could well have been a fully fledged spiral galaxy in the past. There is speculation about the SMC too. It is difficult to justify calling these two anything other than galaxies.

parkner
2013-Nov-09, 07:38 PM
These objects belong to the Milky Way.
http://www.sciencedaily.com/releases/2012/04/120425094352.htm

They should not be treated formally as a galaxies,
like globular clusters of stars, which hardly anyone called galaxies.

Shaula
2013-Nov-09, 07:49 PM
Yes, they do now. And they probably formed locally to it. I could even partly agree with you about the dwarf irregulars/spheroidals. But the LMC/SMC are larger by far and have more hints of structure, the LMC in particular. So they are galaxies. It is rather like saying the smaller partners in multiple star systems should not be called stars.

Jean Tate
2013-Nov-09, 08:00 PM
We have probably about 10^10 big galaxies (similar to the Milky Way or bigger) in sphere of 13.7e9 ly ~ 10 Gly,
then the average distance between galaxies is about 10 Mly;(my bold)

As far as I know, there's no evidence to suggest that the galaxy size distribution function has a real break at around this size (even ignoring what 'size' refers to here - half-light radius? stellar mass? integrated bolometric luminosity?); do you know of any, parkner? I mean, when all the inevitable selection biases are taken into account.

Jean Tate
2013-Nov-09, 08:08 PM
Likely that makes sense to exclude galaxies with less luminosity than the Milky Way, as they are generally not detectable even one billion light years away? Could we detect an isolated galaxy at one billion light years about as bright (magnitude 30 perhaps?) as the Small Magelenistic cloud?

If it were lensed, by a massive foreground galaxy (or cluster), easily.


How much less luminous can a dwarf galaxy be before it considered something else? Sorry the ancient topics are more interesting than the recent topics. Neil

As I said in my reply to parkner, it's hard to unambiguously pick a threshold, demarcating dwarf galaxies from 'ordinary' ones. Perhaps the existence of a SMBH in the center (even if there's no obvious nucleus)? Perhaps the inferred existence of a dark matter halo (this would make 'tidal remnant' galaxies distinct from the rest)?

With SDSS has come the discovery of some extremely faint galaxies; with deeper surveys maybe even fainter ones will be found.

Jean Tate
2013-Nov-09, 08:17 PM
These objects belong to the Milky Way.
http://www.sciencedaily.com/releases/2012/04/120425094352.htm

They should not be treated formally as a galaxies,
like globular clusters of stars, which hardly anyone called galaxies.
M33 may be a satellite of M31, yet it's indistinguishable from a great many other spiral galaxies. Many giant ellipticals in rich clusters have 'satellite galaxies' which are far more massive than our own galaxy or M31; by your logic, we should then not consider even M31 a galaxy (formally)!

There are - AFAIK - lots of 'free floating' dwarf galaxies in the Virgo cluster which are less massive than the most massive of globular clusters, and they're certainly much brighter (especially if you include the biggest GCs seen orbiting other galaxies). Then there's history: Omega Cen (perhaps the biggest of our own galaxy's GCs) is very likely the core of galaxy whose outer parts have been lost due to tidal stripping; go back far enough in the universe's history, and it was likely a perfectly respectable galaxy, possibly even on par with M33.

parkner
2013-Nov-09, 09:07 PM
I do not know any history of the universe, and probably fortunately.
Galaxies have a fairly unique size - like planets and stars.

Hornblower
2013-Nov-09, 09:14 PM
I do not know any history of the universe, and probably fortunately.
Galaxies have a fairly unique size - like planets and stars.

Can you tell us, in appropriate mathematical detail, what you mean by "fairly unique size?"

parkner
2013-Nov-09, 09:31 PM
Of course: size of of complex objects, as well as many other of their parameters, are always pretty well determined by the components and the interactions between them.

Hornblower
2013-Nov-09, 11:50 PM
Of course: size of of complex objects, as well as many other of their parameters, are always pretty well determined by the components and the interactions between them.

How does that figure in the setting of criteria for deciding what types of systems you would choose to classify as galaxies?

ngc3314
2013-Nov-10, 12:23 AM
Planets range in size from Mercury to about 2x Jupiter's diameter, a size range of about 40x. Stars (not counting neutron stars) range from Earth size to about 10 astronomical units (a bit below Jupiter size if you exclude white dwarfs), for a size range of 0.2-2 million. Galaxies have fuzzy edges, but using the common de Vaucouleurs radius defined by surface brightness (not a wonderful comparison but fast) and taking GR8 and NGC 6166 of the top of my head as small and large examples, range from 0.9-70 kiloparsecs diameter at this surface brightness level. These ranges might or might not qualify as "fairly unique" depending a lot on what you want to compare them to.

parkner
2013-Nov-10, 04:08 PM
Pretty good, but the dwarf stars should be again excluded from the stars,
as these dwarf galaxies, which should not be called galaxies.

There are quasars, which are also not galaxies.

Elliptical galaxies also do not specifically fit here.
I guess this is something different from the typical galaxies: flat, sometimes spiral disks or rings at least.

Shaula
2013-Nov-10, 04:17 PM
Quasars are galaxies with active nuclei. Do you also exclude radio galaxies, Seyfert galaxies and so on from this rather unorthodox definition of a galaxy you are using?

parkner
2013-Nov-10, 04:27 PM
We still do not know what it is.
Theoretical wishes are of little use.

Shaula
2013-Nov-10, 04:32 PM
We still do not know what it is.
Theoretical wishes are of little use.
Quasar host galaxies directly imaged:
http://www.eso.org/public/images/eso0529a/
http://www.spacetelescope.org/images/opo9416a/

Even if you have in issue with the unified AGN model, quasars are situated in galaxies.

Jean Tate
2013-Nov-10, 05:01 PM
Pretty good, but the dwarf stars should be again excluded from the stars,
as these dwarf galaxies, which should not be called galaxies.

There are quasars, which are also not galaxies.

Elliptical galaxies also do not specifically fit here.
I guess this is something different from the typical galaxies: flat, sometimes spiral disks or rings at least.
To be blunt parkner, I really have no idea what you're talking about.

Instead of talking in vague generalities, would you please put some numbers to your words?

For example, "but the dwarf stars should be again excluded from the stars": how do you - parkner - decide what's a dwarf star? Please, in an objective manner, which can be used by anyone, and which we'd all agree on the result.

This had me really scratching my head: "Elliptical galaxies also do not specifically fit here". Again, what do you think elliptical galaxies are (I strongly suspect your personal definition is quite different from that used by astronomers)? A fairly recent discovery: a great many - possibly even a majority - of elliptical galaxies in the local universe are 'fast rotators', thus blurring the distinction between them and lenticulars (early-type galaxies with no apparent arms, but distinct disks and bulges).

And "something different from the typical galaxies": by number, dwarf galaxies dominate! Please, take some time to work out what your definitions are, write them up, and post them so that we all can at least have a chance to understand what you're talking about.

Jean Tate
2013-Nov-10, 05:04 PM
We still do not know what it is.
Theoretical wishes are of little use.
OK, so please tell us all parkner, how - from a purely observational perspective - do you distinguish quasars from Seyferts?

parkner
2013-Nov-10, 05:12 PM
Quasar host galaxies directly imaged:
Yes. Quite similar to the Sun in the evening, or it may be thermonuclear explosion in the desert.:p

Hornblower
2013-Nov-10, 05:35 PM
I have no quarrel with what the mainstream astronomers choose to classify as galaxies. Parkner's objections will not affect the progress of astronomy or any branch of science one iota.

Shaula
2013-Nov-10, 05:46 PM
Yes. Quite similar to the Sun in the evening, or it may be thermonuclear explosion in the desert.:p
Not really. More like a host galaxy with a very bright active nucleus at a great distance. Odd that.

parkner
2013-Nov-10, 05:50 PM
For example, "but the dwarf stars should be again excluded from the stars": how do you - parkner - decide what's a dwarf star? Please, in an objective manner, which can be used by anyone, and which we'd all agree on the result.

I have no idea what these objects called dwarf stars;
completely at all, even with which it can be constructed ... I do not know the substance of which the density is thousands of tons per cube.


This had me really scratching my head: "Elliptical galaxies also do not specifically fit here". Again, what do you think elliptical galaxies are (I strongly suspect your personal definition is quite different from that used by astronomers)? A fairly recent discovery: a great many - possibly even a majority - of elliptical galaxies in the local universe are 'fast rotators', thus blurring the distinction between them and lenticulars (early-type galaxies with no apparent arms, but distinct disks and bulges).


I was thinking of mainly the large elliptical galaxies - the type of cD.


And "something different from the typical galaxies": by number, dwarf galaxies dominate! Please, take some time to work out what your definitions are, write them up, and post them so that we all can at least have a chance to understand what you're talking about.

Of course.
There is always a lot of scraps in any production, processing;
according to the popular saying goes: where the wood chopping, there are flying chips and splinters.

ngc3314
2013-Nov-10, 06:40 PM
The Sun is a dwarf star in standard astronomical terminology. Elliptical galaxies are gravitationally bound systems containing stars which come both brighter and fainter than the picturesque spirals, and clearly outnumber them at the luminosity extremes. An effort on my part to inject relevant facts has apparently completely bounced off.

Strange
2013-Nov-10, 07:09 PM
I have no idea what these objects called dwarf stars;

Then how can you hope to exclude them from the set of stars?


I do not know the substance of which the density is thousands of tons per cube.

So are you talking specifically about white dwarfs? (And that would be degenerate matter: http://en.wikipedia.org/wiki/Degenerate_matter)

parkner
2013-Nov-10, 08:41 PM
Then how can you hope to exclude them from the set of stars?

Because there are some sort of 10^12 stars around, mainly made of hydrogen and some other common materrial,
not any fantastic substanse.


So are you talking specifically about white dwarfs? (And that would be degenerate matter: http://en.wikipedia.org/wiki/Degenerate_matter)

It's just an example of the fantastic substance.
Something as moving whole magnetic fields, ropes, tubes, and other shapes in the solar magneto theory, or a real waves without any medium in another... the biggest discovery in early twenty century.

parkner
2013-Nov-10, 08:52 PM
Not really. More like a host galaxy with a very bright active nucleus at a great distance. Odd that.

Of coutrse.
The weak researchers have always found and saw exactly what they wanted to see: the weaker the faster.

Shaula
2013-Nov-10, 09:31 PM
Of coutrse.
The weak researchers have always found and saw exactly what they wanted to see: the weaker the faster.
Right, so they found something the right size, shape, structure and spectrum to be a galaxy, they found a unified model that rather neatly explains a host of AGN phenomena, they tested these theories, refined them and found that they worked well and matched observations made to test the model. Do you have an alternative idea about what they are or are you just generally saying that the scientific method does not work?

parkner
2013-Nov-11, 12:05 AM
Right, so they found something the right size, shape, structure and spectrum to be a galaxy, they found a unified model that rather neatly explains a host of AGN phenomena, they tested these theories, refined them and found that they worked well and matched observations made to test the model. Do you have an alternative idea about what they are or are you just generally saying that the scientific method does not work?

I heard that in cosmology is such a thing as the Copernican principle, and several others.
Considering only this one principle, and compliance with the observations disappears - it is completely zero.


You correctly noticed: in cosmology virtually no scientific methods are used.
We can say that this is the tradition, firmly established historically.

Jean Tate
2013-Nov-11, 12:49 AM
Then how can you hope to exclude them from the set of stars?Because there are some sort of 10^12 stars around,
There are? How do you know that, parkner?


mainly made of hydrogen and some other common materrial,
not any fantastic substanse.

So are you talking specifically about white dwarfs? (And that would be degenerate matter: http://en.wikipedia.org/wiki/Degenerate_matter)

It's just an example of the fantastic substance.
It is?

Consider this (source (http://www.sdss.org/dr5/algorithms/spectemplates/)):

18709

Do you agree that this star seems to be made of hydrogen? Yet it's a white dwarf. How can it be made of hydrogen and also have a very high density, parkner?

Jean Tate
2013-Nov-11, 01:03 AM
Right, so they found something the right size, shape, structure and spectrum to be a galaxy, they found a unified model that rather neatly explains a host of AGN phenomena, they tested these theories, refined them and found that they worked well and matched observations made to test the model. Do you have an alternative idea about what they are or are you just generally saying that the scientific method does not work?I heard that in cosmology is such a thing as the Copernican principle, and several others.
Considering only this one principle, and compliance with the observations disappears - it is completely zero.

Not long ago I asked you, parkner, how you distinguish quasars from Seyferts, purely on the basis of observations.

You did not answer.

I can only guess, of course, why you did not answer, but this response of yours to what Shaula wrote suggests that the reason may be your own ignorance of the very astronomical observations you seem to consider to be so important.

But I'd rather not guess; would you be so kind as to explain what Seyferts have to do with cosmology and the Copernican principle? Oh, and how do you distinguish Seyferts from quasars, purely on the basis of observation?


You correctly noticed: in cosmology virtually no scientific methods are used.
We can say that this is the tradition, firmly established historically.
Ah, so we have another, highly idiosyncratic, definition; namely parkner's, of "the scientific method" (and also, perhaps, of "cosmology")?

Myself, I find it very hard to have an intelligible dialog with someone who seems to have private definitions of just about every key term used in the discussion. From "galaxies" to "dwarf stars" to "fantastic substance" to "the scientific method" to ...

Would you please take some time to think about what you post? And when you do, please try to be extra careful to use words - specific, technical astronomical words - with their usual meanings (or, if not, spell out just what your own, private/idiosyncratic definitions are)?

Thank you in advance.

Shaula
2013-Nov-11, 06:46 AM
You correctly noticed: in cosmology virtually no scientific methods are used.
We can say that this is the tradition, firmly established historically.
Hmm, I talked about how observations led to a hypothesis which turned into a model. From this model a series of quantative tests were devised to see if the hypothesis had predictive power ... What do you call that?

I'm with Jean on this, please define what you regard to be the scientific method. Because from large scale structure formation to stellar life cycles there are huge numbers of examples of scientific method being applied. Astrophysics and cosmology tend to be less precise than, say, condensed matter physics due to the larger uncertainties and lack of ability to perform full lab experiments due to the energies, sizes and timescales invovled. But it is still done in accordance with the scientific method.

tusenfem
2013-Nov-11, 09:38 AM
Of coutrse.
The weak researchers have always found and saw exactly what they wanted to see: the weaker the faster.


parkner you are arguing here against mainstream scientists.
If you want to make such a claims, then you present evidence, and most likely do it in ATM.

Similar as claims like these:


It's just an example of the fantastic substance.
Something as moving whole magnetic fields, ropes, tubes, and other shapes in the solar magneto theory, or a real waves without any medium in another... the biggest discovery in early twenty century.


How is one to read this?

Try to write clearly and understandably and please do not start to question mainstream physics outside of ATM.

You are receiving another infraction.