Some questions about glob. clusters and dark matter

[marcus]

A friend called my attention to an interesting paper by Abe Loeb and David Spergel and somebody else. It seemed to raise unresolved issues about the DM and globular clusters.
Does anybody know the current mainstream thinking about this?

In case anyone is new to this, Loeb and Spergel are two of the top leaders in astronomy in the US---one at harvard and the other at princeton. The fact that they just wrote a paper about this is a hint that there is some interesting problem connected to it.

I don't know very much about this, but I will summarize what I think are the basics.

Globular clusters are roughly (sometimes oblate/elliptical) spheres of a few 100 thousand old stars within a radius of a few tens of light years.

We know they formed early in history because the stars are "metal-poor"---mostly H and He with very little heavier elements.

Milky has about 150 known GC. Some other galaxies have more, it varies.

The GC orbit the galaxy core, passing regularly through the plane of the galaxy. They go on highly elliptical orbits somewhat the way comets orbit the sun. Brief visit in close and then a long time way out, like 50 or 100 thousand LY from core, and then falling back in for another brief pass. Tidal forces tend to pull them apart during their time close to the core. We see them in various stages of disarray. Streamers of stars pulled off of them by tidal force.

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One puzzle is that GC do not seem to have any DM. Typically anyway. So how did they originally form?

AFAIK the current ideas of structure formation say that DM helped ordinary matter to cluster together and form stuff like stars and groups of stars and galaxies and groups of galaxies. There are impressive computer simulations showing how DM helped gather stuff and organize structures.

So did Globular Clusters USED to have Dark Matter that helped them to form into clusters but then later their DM was sucked out of them by tidal disruption? As they passed close to bigger concentrations of mass?

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Also someone suggested that DM might be ANNIHILATED by interaction with ordinary matter. But I don't think that is a mainstream idea. I never heard any theory of DM which would say that it is annihilated that way. But that is a separate question.

 

[MTd2]

This is the paper:

http://arxiv.org/abs/1010.5783

Evidence Against Dark Matter Halos Surrounding the Globular Clusters MGC1 and NGC 2419

Charlie Conroy, Abraham Loeb, David Spergel
(Submitted on 27 Oct 2010)
The conjecture that the ancient globular clusters (GCs) formed at the center of their own dark matter halos was first proposed by Peebles (1984), and has recently been revived to explain the puzzling abundance patterns observed within many GCs. In this Letter we demonstrate that the outer stellar density profile of isolated GCs is very sensitive to the presence of an extended dark halo. The GCs NGC 2419, located at 90 kpc from the center of our Galaxy, and MGC1, located at ~200 kpc from the center of M31, are ideal laboratories for testing the scenario that GCs formed at the centers of massive dark halos. Comparing analytic models to observations of these GCs, we conclude that these GCs cannot be embedded within dark halos with a virial mass greater than 10^6 Msun, or, equivalently, the dark matter halo mass-to-stellar mass ratio must be Mdm/M_*<1. If these GCs have indeed orbited within weak tidal fields throughout their lifetimes, then these limits imply that these GCs did not form within their own dark halos. Recent observations of an extended stellar halo in the GC NGC 1851 are also interpreted in the context of our analytic models. Implications of these results for the formation of GCs are briefly discussed.

This is also interesting:

http://arxiv.org/abs/1010.5783No evidence for a dark matter disk within 4 kpc from the Galactic plane

C. Moni Bidin, G. Carraro, R. A. Mendez, W. F. van Altena
(Submitted on 4 Nov 2010)
We estimated the dynamical surface mass density (Sigma) at the solar Galactocentric distance between 2 and 4 kpc from the Galactic plane, as inferred from the observed kinematics of the thick disk. We find Sigma(z=2 kpc)=57.6+-5.8 Mo pc^-2, and it shows only a tiny increase in the z-range considered by our investigation. We compared our results with the expectations for the visible mass, adopting the most recent estimates in the literature for contributions of the Galactic stellar disk and interstellar medium, and proposed models of the dark matter distribution. Our results match the expectation for the visible mass alone, never differing from it by more than 0.8 $Mo pc^-2 at any z, and thus we find little evidence for any dark component. We assume that the dark halo could be undetectable with our method, but the dark disk, recently proposed as a natural expectation of the LambdaCDM models, should be detected. Given the good agreement with the visible mass alone, models including a dark disk are less likely, but within errors its existence cannot be excluded. In any case, these results put constraints on its properties: thinner models (scale height lower than 4 kpc) reconcile better with our results and, for any scale height, the lower-density models are preferred. We believe that successfully predicting the stellar thick disk properties and a dark disk in agreement with our observations could be a challenging theoretical task.

 

[marcus]

Thanks for providing links!

One thing that arouses my curiosity is this. Correct me if I am missing something but I never heard of DM annihilating except with itself. I've heard a lot about the DM particle being its own antiparticle. So a cloud of DM could radiate X or gamma rays, from rare encounters between the same kind of particle. People look for this wherever they suspect that the DM cloud is especially dense. (So that encounters causing annihil. would happen often enough to be detectable as distinct from background noise.)

So if my impression is correct, then tidal forces would be the main, almost the only, way that a GC could lose its original DM, if it had some to start with.

Maybe someone who has read up on this stuff will clarify this. There has to be some interest to it because spergel and loeb wrote on the topic.

 

[MTd2]

I really don't know about DM. Yes, this is the thing with neutralinos. But, if that was the case, there would be no reason for the center of the galaxy have such undetectable density of dark matter.