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[astro-ph/0508624] The origin of cold dark matter halo densi

Posted: September 02 2005
by Alexia Schulz
Hi All,

Just thought I'd bring this paper to your attention because I think it provides a fairly natural explanation for the universality of dark matter halo profiles, the origin of which has previously been quite murky. The basic idea is that the mass accretion history has two distict phases, first a fast phase dominated by frequent mergers of smaller stuff, and next a slow phase where mass dribbles onto the outskirts of an existing central object.

Rapid changes in the gravitational potential isotropise the velocity field of infalling dark matter during the fast accretion phase, and this leads to the r^-1 shape seen in the central region of dark matter profiles. As the accretion transtions from the fast to slow phase, this isotropisation effect goes away and the profile rolls over to r^-3. If you think of the rollover as the "formation time," this leads naturally to the proper dependance of concentration on formation time (i.e. agrees with N-body sims).

The model they use is quite simple but that's part of their punch line. Using this model with only 2 ingredients they have accurately reproduced a bunch of properties of the CDM halo population, which implies that these ingredients are key to explaining the universal nature of the population.

[astro-ph/0508624] The origin of cold dark matter halo densi

Posted: September 23 2005
by Andrey Kravtsov
Interesting paper and a step in the right direction. It is clear that there are 2 separate phases in halo evolution: early fast accretion period and late quescent accretion period. Transition between the two stages sets scale radius of the halo density distribution, as was shown by Risa Wechsler in her thesis (Wechsler et al. 2002).

There is a key assumption made in this work - that during the fast accretion stage potential fluctuations isotropize particle orbits which leads to \rho\propto r^{-1} profile. However, I don't think this assumption is supported by results of cosmological simulations or results of controlled merger experiments