Giant Galactic Blobs: Uncovering Dark Matter?

[Garth]

The normal IMF (Initial Mass Function) is thought to decrease with decreasing mass below about 0.5 M_Sun. However there is a caveat;
The selection effect may result in the low mass and cool objects being missed.
So the flattening off as in Nereids second link may continue into planetary gas giant sized objects.
So MACHO's may continue in abundance in the IMF below the red dwarf limit and be too small for gravitational lensing detection. Whether or not gas giants could form under these conditions may be better known when we are sure how they form under solar system conditions! (Hot or cold Jupiters?)

We do have evidence of inter-galactic gas in the form of the GGB's, the Lyman alpha forest, the material illuminated by energetic electrons in the radio lobes of active galaxies and quasars.

As I've said before its an open question for me.

Garth

 

[ohwilleke]

Something I've been meaning to do since at least when Garth first posted SCC ... find a recent, good review paper on observational constraints on DM. You know,

1) here's how much DM there is in the solar neighbourhood, inside ~35 kpc in the MW, out to the MW halo, within the LG, near the LMC and SMC, near M31, near M33, near the LG dwarfs, within other nearby groups (e.g. M81), within nearby clusters (e.g. Virgo, Fornax), near the large and small galaxies in these clusters, within nearby superclusters, ... and how it is distributed

Along this line, McGaugh (a MOND guy) has a paper describing constraints on the distribution of DM is you say MOND is wrong on DM is right. It is here:

http://arxiv.org/abs/astro-ph/0403610

Stacy S. McGaugh (University of Maryland), "The Mass Discrepancy-Acceleration Relation: Disk Mass and the Dark Matter Distribution" (Accepted for publication in the Astrophysical Journal appearing at Astrophys.J. 609 (2004) 652-666).

The mass discrepancy in disk galaxies is shown to be well correlated with acceleration, increasing systematically with decreasing acceleration below a critical scale a0 = 3700 km^2/s^2/kpc = 1.2E-10 m/s/s. For each galaxy, there is an optimal choice of stellar mass-to-light ratio which minimizes the scatter in this mass discrepancy-acceleration relation. The same mass-to-light ratios also minimize the scatter in the baryonic Tully-Fisher relation and are in excellent agreement with the expectations of stellar population synthesis. Once the disk mass is determined in this fashion, the dark matter distribution is specified. The circular velocity attributable to the dark matter can be expressed as a simple equation which depends only on the observed distribution of baryonic mass. It is a challenge to understand how this very fine-tuned coupling between mass and light comes about.

The big take home message is that even if you accept DM, it has far more observational constraints than the naiive version of DM theory would suggest. Observed DM distributions inferred from dynamical data in hundreds of galaxies are closely coupled to luminous matter distributions in those same galaxies according to a MOND like formula.

Citations to this article are shown here: http://citebase.eprints.org/cgi-bin/citations?id=oai:arXiv.org:astro-ph/0403610

Most notable is this one: http://arxiv.org/PS_cache/astro-ph/pdf/0409/0409239.pdf which looks at a lot of different fitting approaches and comes away inconclusive.

 

[Nereid]

Is anyone going to take a stab at being quantitative (re the mass function of 'stars' in inter-galactic space in rich clusters)?

 

[Garth]

Nereid a very good exercise!
OOM only. Feel free to disagree or correct my values - (thankfully) I know there is no need to say that!

Postulate: DM is (largely?) baryonic in the form of dim condensed objects, Black Holes, red dwarfs, Jupiters and smaller planetismal type bodies condensed out of primordial gas with 'Freely Coasting BBN' high metallicity, i.e. some oxygen, nitrogen and carbon so we have some ice and hydrocarbons to make 'snowballs'.

Mass of MW Halo within 100 kpc of MW centre ~ 5.5x10¹¹M_Sun

What are these MACHO's?
Some black hole/red dwarf type MACHO's have been detected, but they would account for less than 10%DM.

So assume the rest are smaller objects, how many do we need and ho dense are they?

If 'Jupiters' of 0.001M_Sun we need:
5x10¹¹/{(4/3)pi10¹⁵x10³} /psc³ = 0.1/psc³.

Now the Oort cloud is thought to be about 0.1 M_Jupiter (OOM!) so we are talking about one Oort cloud of icy bodies in every cell represented by the volume between here and Proxima Centuri, or one Jupiter every two parsecs, or one (~3M_Sun) black hole every 30 parsecs.

Problems: Not enough detected larger MACHO's, so 5% in form of (say) Black Holes, but not enough 'metals' for much ice, so we are left with Jupiters of primordial hydrogen, helium and some 'metals'.
How would such Jupiter objects gravitationally condense? I do not know, but discoveries of extra-solar planetary systems suggest there are more of them than we first thought. Jeans Mass for IGM at around 10⁰K and typical IGM densities requires galactic cluster masses. However they condensed down into smaller objects, and that process is still very obscure, perhaps most of the mass went into free Jupiters? Beyond this I am 'stabbing in the dark' and better stop!

Of course some matter would remain as gas, which is responsible for the Lyman alpha forest and the radio-emitting medium illuminated by galactic/quasar jets.
What density is required in the deep IGM to account for these?

There is another possibility. The "DM is baryonic" conclusion comes from the Freely Coasting cosmological model, perhaps there are two explanations for DM, one for cosmological (i.e. IGM DM) and another explanation for galactic DM, such as a MOND type modification to Newtonian gravity.

Garth

 

[Chronos]

I hate to go out on a limb, but could intervening matter have a gravitational shielding effect? I'm thinking along the lines of gravitational lensing. Spacetime distortion is more concentrated in-line, would it not be weaker in the 'shielded' region? Global effects would be the same, but local effects would be more intense in the focal zone, and less intense along normal geodesic lines. Would this explain the allias effect? This would be pretty easy to test since we know the mass and distance of the moon.

 

[ohwilleke]

If there is a shielding effect, one would expect that it would be pretty localized and not have a big effect on large scale dynamics. Why? Because most of the universe is empty space. Even in a typical galaxy, there isn't much line of sight/geodesic line interference.

Also, given the small OOM of the proposed allais effect, and the huge masses necessary to produce the effect (planetary), gravitational shielding seems like it would be pretty much a curiosity only.

 

[turbo]

Good points, ohwilleke, however if the Allais effect is experimentally verified, it will by no means be a curiosity. A very basic and important implication of the Allais effect (if proven) is that the mechanics of gravitation is inadequately described by GR. GR says that masses warp space-time. If the gravitational effect of the Sun, as measured at any place on the Earth can be moderated by placing our very modest-sized moon directly between the Sun and that location (solar eclipse), then we will have to admit that the GR mathematical model of space-time curvature is only a handy approximation, and is not "real" in and of itself. The shielding effect proposed by Allais begs for a true mechanical (kinetic/dynamical) model of gravitation, which is where LQG is currently stuck - trying to reconcile flat-field quantum physics with the curved space-time of GR.

I will shut up now, lest I be accused of hand-waving by those much wiser and more perceptive than myself.

 

[ohwilleke]

A very basic and important implication of the Allais effect (if proven) is that the mechanics of gravitation is inadequately described by GR.

Good point. And, of course, new physics is always interesting, even when it isn't very useful.

 

[ohwilleke]

Along this line, McGaugh (a MOND guy) has a paper describing constraints on the distribution of DM is you say MOND is wrong on DM is right.

Another paper in a similar vein is here: http://cul.arxiv.org/abs/astro-ph/0406487

 

[Chronos]

If there is a shielding effect, one would expect that it would be pretty localized and not have a big effect on large scale dynamics. Why? Because most of the universe is empty space. Even in a typical galaxy, there isn't much line of sight/geodesic line interference.

Also, given the small OOM of the proposed allais effect, and the huge masses necessary to produce the effect (planetary), gravitational shielding seems like it would be pretty much a curiosity only.

I agree entirely. I can't picture how such an effect would have any large scale consequences. I was merely thinking it might be interesting to see what, if anything, would happen if you plugged the effect into an N-body simulation - assuming of course the effect is real and could be quantified.