Non-baryonic cold dark matter ("CDM"), the observational evidence

[sol invictus]

As it does away with CDM entirely, it is also, by default, good at explaining the MW halo microlensing observations.

Can you really say that? Vanilla MOND isn't a complete theory and doesn't make any predictions for lensing - i.e. it might affect lensing by ordinary matter, or it might not.

One should also mention the Bullet cluster observation, which conflicts very sharply with the MOND prediction.

 

[DeiRenDopa]

As it does away with CDM entirely, it is also, by default, good at explaining the MW halo microlensing observations.

Can you really say that? Vanilla MOND isn't a complete theory and doesn't make any predictions for lensing - i.e. it might affect lensing by ordinary matter, or it might not.

.
No, not really (I can't say that) ... it was an extreme summary.

More fully: being classical, MOND cannot and does not reproduce GR 'lensing', period (nor does any proponent claim that it can, should, or does).

The MW halo microlensing observations can be interpreted in terms of bounds on the mass in the MW halo in the form of MACHOs (massive compact halo objects), principally low-mass stars. This estimated (MACHO) mass is consistent with a MOND analysis of the MW rotation curve (several papers on this), though the analysis is quite tricky (basically because we are inside the MW).
.

One should also mention the Bullet cluster observation, which conflicts very sharply with the MOND prediction.

Indeed.

I intend to cover that when I get to point 2 (observational evidence for CDM in rich clusters of galaxies) and also address how well MOND (and MOND extensions) fares with respect to cosmology, when I get to point #3 ... patience please!

 

[robinson]

Not sure which Dark matter topic this would go in, but there is Dark Matter stuff here:
http://astro.uchicago.edu/cosmus/
http://astro.uchicago.edu/cosmus/projects/evolution/
Dark Matter simulation and movies. Interesting stuff.

 

[Wangler]

Just one or two counter points here:

The main technique used to estimate the distribution of mass in spiral galaxies, as a function of radius from the centre (nucleus) is to derive a rotation curve from observations of the light (radio, etc; electromagnetic radiation - I'll use 'light' as a synonym) emitted by such galaxies.

One technique involves observing how the line of sight velocity changes with position; here is a concise but accurate summary of how it's done, using a 'long slit' spectrum, and emission lines in the visual waveband.

There are many other ways to obtain a spiral galaxy rotation curve, using different wavebands, different lines; integrated light from sizable chunks of the target galaxy, individual sources (e.g. HII regions, bright stars); and so on.

The results are the same: the curves either keep rising or flatten out, right out (radially) to where no more light seems to be coming from the galaxy.

Using textbook physics, these curves can be interpreted to mean that the mass 'closer in' to the centre of the galaxy (than at any radius) keeps increasing as the radius increases. In fact, no other standard physics textbook interpretation has been proposed, that is also consistent with all the relevant observations.

It is my understanding that the expected rotation curves would peak at some value (say in the galactic disk, at a radius << the radius at which the surface brightness decreases to a background value), and then slowly decrease to zero.

This expected rotation curve would be generated by a massive core, and somewhat less massive spiral arms, as opposed to a strictly uniform mass distribution.

Now, the observed rotation curves behave exactly as DRD says; they do not slowly decrease, but rather they increase or remain constant.

The concordance opinion is that this is due to a large, massive halo of dark matter surrounding the galaxy.

Now, DM is assumed to interact with visible matter (VM) primarily via gravity.

However, the mass distribution of DM that would provide the observed rotation curves, in conjunction with the known VM mass in the core and disk, has mass density that either remains constant for increasing radius, or possibly increases with increasing radius.

This creates a little bit of a problem, as it would be expected that the DM halo, under gravity alone, would have a mass distribution that would have greater density at the center.

If I am not mistaken, we have to make very specific assumptions about the dynamics of the halo to avoid discrepancies with observations of radial velocity close to the galactic core.

Beyond the faintest (integrated light) edges of galaxies are objects which are moving within the gravitational well of the galaxies. These objects include planetary nebulae, globular clusters, and satellite (dwarf) galaxies.

It may be interesting to note that while the globular clusters' motion around our galaxy do apparently support a DM halo, the internal globular rotational dynamics do not.

=DeiRenDopa;3661266]..........elliptical galaxies, which do not have rotation curves.

I think you meant to say that elliptical galaxies have rotation curves, but they aren't measurable to the radial extent than rotation curves for spirals can be.

The density of this (dark) halo mass decreases with radial distance from the nucleus.

As I mentioned before, the mass density is not at it's maximum at the center of the halo, however. I find this troubling from a dynamical point of view, as it seems to require some sort of self-interacting DM.

 

[DeiRenDopa]

This is a very big topic Wrangler; I applaud you for taking the time to try to understand it!

Looking at just two things, for now:

... snip ...

Beyond the faintest (integrated light) edges of galaxies are objects which are moving within the gravitational well of the galaxies. These objects include planetary nebulae, globular clusters, and satellite (dwarf) galaxies.

It may be interesting to note that while the globular clusters' motion around our galaxy do apparently support a DM halo, the internal globular rotational dynamics do not.

I think you misunderstood what you read ...

The internal motions of globular clusters have nothing to do with any DM halos of the galaxies they are in orbit around.

From the motions of globular cluster stars you can conclude that the (radial) mass distribution is consistent with the estimated mass distribution due to the observed stars alone (actually you have to extrapolate to include the stars too faint to be detected). In other words, there is (apparently) no CDM in globular clusters.

This is one very interesting result!

However, it is beyond the scope of what I want to cover in this thread (observational evidence for CDM).
.

.........elliptical galaxies, which do not have rotation curves.

I think you meant to say that elliptical galaxies have rotation curves, but they aren't measurable to the radial extent than rotation curves for spirals can be.

... snip ...

Again, I think you have misunderstood ...

Elliptical galaxies do not have rotation curves, because the stars (and what little gas and dust there is) do not move about the centre (of mass) with the kind of common motion found in spiral galaxies (the same is true of globular clusters and (many) dwarf galaxies).

If you could trace the orbits of a million random stars in an elliptical, you'd find they go every which way; as a whole, elliptical galaxies have essentially zero net angular momentum (unlike spirals).

What I think you are referring to is the 'velocity dispersion' - along any line of sight, the distribution of the speeds of the stars (in the elliptical).

 

[sol invictus]

However, the mass distribution of DM that would provide the observed rotation curves, in conjunction with the known VM mass in the core and disk, has mass density that either remains constant for increasing radius, or possibly increases with increasing radius.

That is not correct. Basic Newtonian mechanics is enough to tell you that the mass density in the halo should decrease like 1 over radius squared to provide a flat rotation curve:

[latex]$m v^2/r = G m M(r) /r^2 \rightarrow M(r) \propto r \rightarrow \rho(r) \propto 1/r^2$[/latex]

 

[robinson]

Elliptical galaxies do not have rotation curves, because the stars (and what little gas and dust there is) do not move about the centre (of mass) with the kind of common motion found in spiral galaxies (the same is true of globular clusters and (many) dwarf galaxies).

Incorrect.

 

[sol invictus]

Incorrect.

No, Robinson.

DRD is correct (as usual), and you are wrong in a particularly smarmy way (as usual).

 

[robinson]

Most elliptical galaxies have very little rotation, but some have significant velocity gradients along their optical major axis.

See
Rotationally supported Virgo dwarf elliptical galaxies
L. van Zee , E.D. Skillman and M.P. Haynes

The Proceedings of the International Astronomical Union (2005), 1: 68-72

 

[robinson]

By all means, just keep saying stuff, without offering a reputable source for the information. It is amusing.

Here is another paper that might be on topic.

Galaxy Rotation Curves Without Non-Baryonic Dark Matter
http://arxiv.org/abs/astro-ph/0506370

I bring it up because it mentions the rotation of elliptical Galaxies.

 

[DeiRenDopa]

Most elliptical galaxies have very little rotation, but some have significant velocity gradients along their optical major axis.

See
Rotationally supported Virgo dwarf elliptical galaxies
L. van Zee , E.D. Skillman and M.P. Haynes

The Proceedings of the International Astronomical Union (2005), 1: 68-72

Indeed.

It's difficult to write just a few words, as a summary, and also cover all bases, and you can find lots of examples in my posts (you don't even have to try very hard). Earlier, for example, I said "As it does away with CDM entirely, it is also, by default, good at explaining the MW halo microlensing observations"; sol called me on this (rightly), it was terse to the point of being highly misleading.

So too with the paper you cite - rotation has been observed in some elliptical galaxies.

But wait! There's more!!

Here is what I actually wrote:

Elliptical galaxies do not have rotation curves, because the stars (and what little gas and dust there is) do not move about the centre (of mass) with the kind of common motion found in spiral galaxies (the same is true of globular clusters and (many) dwarf galaxies).

Hmm ...

I plead guilty your (robinson) Honour; I did not, in this sentence, clearly distinguish (normal, or giant) ellipticals from dwarf ellipticals.

In my defence, however, I would like to point out that I knew that observations of some dwarf galaxies can be interpreted as rotation curves, and that I knew these some included certain dwarf ellipticals (in the Virgo cluster, no less) ... that's one reason* why I used "(many)" to qualify "dwarf galaxies".

These kinds of clarifications and caveats are many, given the complexity and richness of the observations. To give just one other example: there is a class of galaxy called 'lenticulars'. They look like spiral galaxies without (much) gas or dust, but they do, generally, have rotation curves. However, from some viewing angles it is difficult to distinguish an elliptical from a lenticular, especially if the resolution is also low, so some galaxies classified as ellipticals may, in fact, be lenticulars!

Perhaps a more fruitful approach to this might be for you to ask for clarification, rather than respond with a bald 'Incorrect'?

Or should I add, for your benefit, some boilerplate caveat before almost everything I post?

* another is that some dwarf galaxies, not dwarf ellipticals, show clear rotation curves - there is a class of dwarf galaxies called 'dwarf spirals', for example.

 

[Wangler]

This is a very big topic Wrangler; I applaud you for taking the time to try to understand it!

Well, thanks for starting the thread, and giving us a forum to discuss!

Looking at just two things, for now:
I think you misunderstood what you read ...

The internal motions of globular clusters have nothing to do with any DM halos of the galaxies they are in orbit around.

From the motions of globular cluster stars you can conclude that the (radial) mass distribution is consistent with the estimated mass distribution due to the observed stars alone (actually you have to extrapolate to include the stars too faint to be detected). In other words, there is (apparently) no CDM in globular clusters.

This is one very interesting result!

However, it is beyond the scope of what I want to cover in this thread (observational evidence for CDM).

Well, I think that the fact that these large associations that are called globular clusters, some of which are nearly the size of a dwarf spherical galaxies (Omega Centauri, for example), do not show evidence for dark matter in their stellar rotational profiles is a piece of observational evidence that should be considered in a critical assesement of the evidence as a whole.

Again, I think you have misunderstood ...

Elliptical galaxies do not have rotation curves, because the stars (and what little gas and dust there is) do not move about the centre (of mass) with the kind of common motion found in spiral galaxies (the same is true of globular clusters and (many) dwarf galaxies).

If you could trace the orbits of a million random stars in an elliptical, you'd find they go every which way; as a whole, elliptical galaxies have essentially zero net angular momentum (unlike spirals).

What I think you are referring to is the 'velocity dispersion' - along any line of sight, the distribution of the speeds of the stars (in the elliptical).

Indeed, I think I did misunderstand this concept. Thanks for the clarification.

Can we look at the 'velocity dispersion' and draw conclusions, or do we just rely on the other observations that you mention for ellipticals?

 

[DeiRenDopa]

By all means, just keep saying stuff, without offering a reputable source for the information. It is amusing.

.
If that's what you want, by all means simply ask for it ... do you think it is not available?

One reason I am puzzled by this comment of yours is that when I took the trouble, in other threads, to provide such "reputable source(s)", in response to your (generally good) questions, you seem to have simply ignored them.

Another, more pertinent, reason for not providing such is that, based on what you have written about your own training and abilities to read and understand physics (etc) papers, I suspect you won't get much from any such source.

Would you mind clarifying please?
.

Here is another paper that might be on topic.

Galaxy Rotation Curves Without Non-Baryonic Dark Matter
http://arxiv.org/abs/astro-ph/0506370

I bring it up because it mentions the rotation of elliptical Galaxies.

Indeed.

But did you actually read this paper, robinson?

Here's what it says, in the caption for Figure 4 (to take just one relevant example; I added bolding):

Rotation curve for the elliptical galaxy NGC 3379. Both rotation curves are the same best fit to a parametric mass distribution (independent of luminos1ity observations) – a two parameter fit to the total galactic Mass, M, and a core radius, rc. The red points (with error bars) are samplings of the circular velocity profile constrained by orbit modeling according to Romanowsky et al. (2003, 2004). The solid black line is the rotation curve determined from MSTG, the dash-dot cyan line is the rotation curve determined from MOND.

And what do Romanowsky et al. (2003, 2004) have to say, concerning the observations they made?

Here's the first part of the caption from Figure 4 of the 2003 paper (just an example; I added bolding):

Line-of-sight velocity dispersion profiles for three elliptical galaxies, as a function of projected radius in units of the effective radius.

Hmm ...

Perhaps you could explain for all readers, robinson, how Brownstein and Moffat were able to derive a rotation curve from Romanowsky et al. (2003)'s line-of-sight velocity dispersion profiles (or the underlying raw data)?

Or, maybe, just maybe, you did some searching (using Google?), turned up some phrases that seemed to contradict what I had written, and just posted merrily away ... without understanding the underlying physics, must less what the actual papers you found were saying?

This really doesn't become you robinson; quote mining is a tactic more suited to explicitly anti-science folk like ID proponents (or Zeuzzz) than a cynical sceptic ... at least, that's my opinion.

 

[DeiRenDopa]

Well, thanks for starting the thread, and giving us a forum to discuss!

.
You're welcome.

I should add that if you are interested in diving deeper you might consider going to BAUT (http://www.bautforum.com), which is run by Phil Plait (well known to JREF regulars, I think) and Fraser Cain (he of Universe Today fame). There are quite a few professional astronomers there, and they have an excellent Q&A section.

If you want to get into the physics, you might consider going to PhysicsForums (http://www.physicsforums.com), in addition to taking classes. I was going to suggest this in answer to the other parts of your post (as sol has said, you misunderstood what I wrote - perhaps I wasn't clear enough? - and you misunderstand the relevant, Newtonian, physics).
.

Looking at just two things, for now:
I think you misunderstood what you read ...

The internal motions of globular clusters have nothing to do with any DM halos of the galaxies they are in orbit around.

From the motions of globular cluster stars you can conclude that the (radial) mass distribution is consistent with the estimated mass distribution due to the observed stars alone (actually you have to extrapolate to include the stars too faint to be detected). In other words, there is (apparently) no CDM in globular clusters.

This is one very interesting result!

However, it is beyond the scope of what I want to cover in this thread (observational evidence for CDM).

Well, I think that the fact that these large associations that are called globular clusters, some of which are nearly the size of a dwarf spherical galaxies (Omega Centauri, for example), do not show evidence for dark matter in their stellar rotational profiles is a piece of observational evidence that should be considered in a critical assesement of the evidence as a whole.

OK, I'm confused ...

Whether globular clusters show evidence for CDM or not is surely irrelevant to the observational evidence showing that galaxies, in general, do?

I mean, we don't demand that globular clusters show evidence of dust and PAH just because spiral galaxies do, do we? Ditto SMBH (super-massive black holes).

Oh, and in light of the difficulty robinson is having with understanding what I write (and you too), would you please avoid the term "stellar rotational profiles" when talking about globular clusters? Or at the very least define it in such a way to link it with 'velocity dispersion', or some other standard term?
.

Indeed, I think I did misunderstand this concept. Thanks for the clarification.

Can we look at the 'velocity dispersion' and draw conclusions, or do we just rely on the other observations that you mention for ellipticals?

Yes, you can ... but it's very tricky to do ...

For starters, the parts of ellipticals (not dwarf ellipticals!) that have sufficient surface brightness for reliable velocity dispersions to be observed are not, generally, expected to be dominated by CDM (the stars etc are the dominant form of mass there). Then tracers of gravitational potential ('well') beyond the visible edge - such as planetary nebulae or satellite galaxies - are rather few, the observations difficult, and the interpretations subject to many uncertainties.

The Romanowsky et al. (2003, 2004) papers are landmark ones; the 2003 one for example has been cited by 138 others (according to ADS); their conclusions have been subject to much scrutiny (which is ongoing) ...

 

[Wangler]

That is not correct. Basic Newtonian mechanics is enough to tell you that the mass density in the halo should decrease like 1 over radius squared to provide a flat rotation curve:

[latex]$m v^2/r = G m M(r) /r^2 \rightarrow M(r) \propto r \rightarrow \rho(r) \propto 1/r^2$[/latex]

But, wouldn't we expect a density decreasing in proportion to the square of the radius, for a halo of self-gravitating matter?

Just as you show!

But, this implies that the density ramps up quickly as r->0. That doesn't match what we see in the rotation curves, is it? Doesn't the effect of DM "peter out" as r gets smaller? Is there a simple explanation for that?

 

[Wangler]

OK, I'm confused ...

Whether globular clusters show evidence for CDM or not is surely irrelevant to the observational evidence showing that galaxies, in general, do?

I mean, we don't demand that globular clusters show evidence of dust and PAH just because spiral galaxies do, do we? Ditto SMBH (super-massive black holes).

I guess your point is correct. The fact that globular clusters do not show evidence for DM in their velocity dispersion (and I mean that this indicates they have little or no DM associated with them) has no bearing on the observational evidence showing that galaxies, in general, do.

Rather, it points to a different question concerning the reasons why one association of matter (globs) behave differently than others (dwarfs).

Obviously, there are some scale differences, as well as compositional differences. But, I would expect these differences to play a minor role when talking about something like DM that is beholden to none but gravity.

Oh, and in light of the difficulty robinson is having with understanding what I write (and you too), would you please avoid the term "stellar rotational profiles" when talking about globular clusters? Or at the very least define it in such a way to link it with 'velocity dispersion', or some other standard term?

That's what you get for engaging a layman in discussions of this nature!

This layman thanks you for having this discussion, again!

Yes, of course, I will try to be better in my terminology in this regard.

I have also heard a term associated with velocity dispersion, that helps to further describe the one around a spiral (very ordered, many stars moving in the same direction, etc) and the one around an elliptical, for example.

I cannot find that terminology again; would it be a "warm velocity dispersion" for an elliptical, and a "cool velocity dispersion" for a spiral, or something like that? Do you know what I am talking about?

 

[sol invictus]

I'm confused. You said:

However, the mass distribution of DM that would provide the observed rotation curves, in conjunction with the known VM mass in the core and disk, has mass density that either remains constant for increasing radius, or possibly increases with increasing radius.

This creates a little bit of a problem, as it would be expected that the DM halo, under gravity alone, would have a mass distribution that would have greater density at the center.

That's incorrect, as I have just shown. To produce a flat rotation curve (a velocity which doesn't depend on radius) one needs a density that falls of like 1/r^2, not a density that grows or remains constant.

So I don't understand your next comment:

But, wouldn't we expect a density decreasing in proportion to the square of the radius, for a halo of self-gravitating matter?

Just as you show!

I'm not sure what we should expect for the density - that depends on initial conditions and interactions. Certainly I'm not aware of any simple or general argument that predicts 1/r^2 - are you? But that's what we need in order to explain the observed flat rotation curves.

 

[Wangler]

I'm confused. You said:



That's incorrect, as I have just shown. To produce a flat rotation curve (a velocity which doesn't depend on radius) one needs a density that falls of like 1/r^2, not a density that grows or remains constant.

So I don't understand your next comment:



I'm not sure what we should expect for the density - that depends on initial conditions and interactions. Certainly I'm not aware of any simple or general argument that predicts 1/r^2 - are you? But that's what we need in order to explain the observed flat rotation curves.

Sorry, Sol, I can see why my post confused you. I have since edited, with another comment regardign the dependence of density on radius.

I thought that a self-gravitating sphere of massive particles, when equilibrium is achieved, would have a density that is proportional to 1/r^2.

Not valid for the singularity at r=0, though.

 

[Wangler]

cold gas: no; no 21 cm emission (from H), the spectra of distant sources show no 'local' (absorption) lines

Hey, don't some people, who think that EM could explain the galaxy rotation curves, assume these gas clouds are subject to EM forces, because they are ionized, or charged? DIscounting any polar effects due to the charge distribution in the H atom/molecule itself.

But, isn't the 21cm emission from neutral hydrogen, so EM fields wouldn't affect this gas the same way it would affect truly ionized gas?

 

[sol invictus]

I thought that a self-gravitating sphere of massive particles, when equilibrium is achieved, would have a density that is proportional to 1/r^2.

Why did you think that?

As you point out, such a density profile is obviously not valid at small r. But it is equally obviously not valid at large r (because otherwise the total mass would be infinite). So at most such a behavior could be valid over an intermediate range of radii (and evidently is, or close, for DM halos).

As I said, I'm not aware of an explanation of why that should be the profile (other than the results of simulations), but perhaps there is one?