Current galaxy formation in the CDM model

[hellfire]

The CDM model predicts the existence of lots of dark halos without star formation. These are halos which did not attract enough baryons in order to set some kind of gravitational instability, which could lead to the further collapse of the baryons and to star formation (see, e.g. http://arxiv.org/abs/astro-ph/0502312). On the other hand, mergers of halos and galaxies are still ongoing within galaxy clusters.

I assume that collisions of halos may lead, in some cases, to some kind of energy loss or dissipation (e.g. angular momentum loss or shocks) of the baryonic gas, inducing a gravitational instability (fragmentation) and perhaps leading to a first star formation.

If we consider that the birth of a galaxy is the formation of it’s first stars, this would mean that galaxy formation is a process which is currently onging. However, it is usually claimed that galaxy formation does not take place currently, or, at least, that it would be surprinsing to find newly formed galaxies in the neighborhood, see e.g. http://hubblesite.org/newscenter/newsdesk/archive/releases/2002/16/text/

So, what about my assumption above?

 

[Garth]

One question with the use of the term 'dark halos' is whether we are talking about a dark baryonic halo or non-baryonic halo. In your link the paper was referring to an HI region, and obviously we would not be able to detect a non-baryonic DM halo without some stars as a tracer for its gravitational potential well, but which exactly is being predicted by the CDM model in this respect?

I believe the CDM model requires the existence of lots of non-baryonic DM in dark halos to encourage the condensation of sub-Jeans mass baryonic clouds into galaxies etc. That process could still be going on today.

The observation linked to is that of an HI region of galactic mass in the IGM of the Virgo cluster. Obviously there is a lot of baryonic matter out there as well as any non-baryonic. What, I wonder, would be the contribution of these dark baryonic HI regions to the total cosmological density parameter?

Garth

 

[hellfire]

but which exactly is being predicted by the CDM model in this respect?

I believe the CDM model requires the existence of lots of non-baryonic DM in dark halos to encourage the condensation of sub-Jeans mass baryonic clouds into galaxies etc.

Yes, this is also my understanding. There should be a high number of dark halos with sub-Jeans mass baryonic clouds bound to them. May be some of them have no baryonic content at all. As far as I understand, this is due to the fact that the primordial power spectrum leads to density perturbations of any scale in the non-baryonic dark matter. After recombination baryons will flow into these potential wells. However, there is a lower limit for the mass of a baryonic cloud to start gravitational collapse (the Jeans mass of about 10⁵ solar masses after recombination). Today there should be still halos without sufficient baryonic content for gravitational instability and collapse. In my opinion (initial) starbursts could be also triggered due to mergers and this might be possible also today.

 

[Nereid]

Here's a thought: that there are a fair number of compact objects in the intra-cluster medium now has a firm observational basis (well, at least in the Virgo cluster). If we could do some really detailed work on the kinematics of these - PNe, RGB stars, ... - would the (radial) velocity data enable us to 'detect' blobs of DM which are essentially baryon-free? Or would it simply reveal a chaotic mess of shredded dwarfs and GCs, tidal tails, and tracers of H clouds?

 

[Garth]

A good thought Nereid but as luck would have it - a "chaotic mess"?

Garth

 

[Nereid]

Now here's another thought ... the HUDF was of a piece of sky deliberately chosen to be free and clear of anything significant, locally.

The HST also took a deep look at an outskirts field of M31.

What about equally deep peeks, using ACS (which has a still tiny FOV, but maybe, just maybe useful), of selected fields in nearby groups (thus avoiding severe LG undersampling problems, among others), e.g. M81? Maybe the footprints of DM blobs could be seen?

Alternatively, if LSST (or alternatives, POI?) get going, we will have SDSS-like data, down to ~Bmag 24 (? I'm going from memory) - comprehensive, multi-fibre spectral work in selected fields may collect enough data to allow a first (statistical) assessment of (IGM) space densities, no?

 

[ohwilleke]

Here's a thought: that there are a fair number of compact objects in the intra-cluster medium now has a firm observational basis (well, at least in the Virgo cluster). If we could do some really detailed work on the kinematics of these - PNe, RGB stars, ... - would the (radial) velocity data enable us to 'detect' blobs of DM which are essentially baryon-free? Or would it simply reveal a chaotic mess of shredded dwarfs and GCs, tidal tails, and tracers of H clouds?

Doesn't it follow that the absence of such blogs in the intra-culster medium, and to my knowledge they haven't been found, would, if not falisfying DM, at least, throw a spanner in the works?

 

[Nereid]

Doesn't it follow that the absence of such blogs in the intra-culster medium, and to my knowledge they haven't been found, would, if not falisfying DM, at least, throw a spanner in the works?

Not necessarilly.

First, there have been a few (one?) cluster DM maps made (and the other name for the proposed LSST is 'Dark Matter Telescope' ), and they show much the same as what the (admittedly marginal) DM results from SZE observations, and X-ray profiles, show - a broad DM distribution, more or less centred on the cluster cD galaxy (or galaxies).

Second, AFAIK, no one has done this sort of thing (there is a paper that reports observations using PNe as tracers, for an NGC galaxy; they report 'no DM'. However, that was one galaxy, not ICM) ... and it would be quite a tour de force to pull such a study off.

Third, who knows what the ICM contains and how it is distributed and interacts (no Garth, not at the 1-10 Mpc level; at the 1 kpc level, or finer)? For example, look at all the structure which SDSS has turned up so far - streams, shredded globulars, LSB 'blobs', ... and look at what Subaru found when it took a deep look at a 'primordial' LG dwarf (all sorts of evidence of evolution), ... and what does the outskirts of M31 look like? ... and remember those outer 'shells' they found around ellipticals (what, 20 years ago now?) ...

 

[wstevenbrown]

Nereid: I would like your comments on this one.

http://arxiv.org/astro-ph/0506753

It seems to discount both prominent CDM halo models, at least for Low Surface-Brightness Galaxies. At first I thought the sample might have been dominated by recent impacts/mergers, leaving a non-virialized halo-- that turned out not to be the case. It may be, then, that the reason that LSB's are so late in developing is that they had no DM to assist them in clumping; i.e., they may be fully baryonic matter-dominated, giving us a peek at what the universe might be like without DM.

This is not a wholesale condemnation of DM, but only one instance where the CDM models may have been misapplied.

Best regards-- Steve

 

[Chronos]

But the whole surface brightness issue raises more problems for baryonic presence than the LCDM model. References to good observational evidence is necessary to sway me on that count. I'm perfectly willing to entertain that possibility, but not based on 'hand waving' objections... which is exactly what that paper promotes.

 

[wstevenbrown]

"... and look at what Subaru found when it took a deep look at a 'primordial' LG dwarf (all sorts of evidence of evolution)"

Were you perhaps referring to the Ursa Minor dwarf spheroidal? It seems to have two nuclei of unequal size, with a central void which is not a superbubble. Stellar populations and spatial distributions indicate a merger ca. 3 Gyr ago.

Or did I miss something? S