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[astro-ph/0508624] The origin of cold dark matter halo density profiles
 
Authors:Yu Lu, H.J. Mo, Neal Katz, Martin D. Weinberg
Abstract:N-body simulations predict that CDM halo-assembly occurs in two phases: 1) a rapid accretion phase dominated by major mergers with a rapidly deepening potential well; and 2) a slow accretion phase characterised by a gentle addition of mass to the outer halo with little change in the inner potential well. We demonstrate, using one-dimensional simulations, that this two-phase accretion leads to CDM halos of the NFW form and provides physical insight into the properties of the mass accretion history that influence the final profile. During the fast accretion phase fluctuations in the gravitational potential effectively isotropise the velocities of CDM particles and we show that this leads to an inner profile $\rho(r)\propto r^{-1}$. Slow accretion onto an established potential well leads to an outer profile with $\rho(r)\propto r^{-3}$. The concentration of a halo is determined by the fraction of mass that is accreted during the fast accretion phase. Using an ensemble of realistic mass accretion histories, we show that the model predictions of the dependence of halo concentration on halo formation time, and hence the dependence of halo concentration on halo mass, and the distribution of halo concentrations all match those found in cosmological N-body simulations. Using a simple analytic model that captures much of the important physics we show that the inner $r^{-1}$ profile of CDM halos is a natural result of hierarchical mass assembly with a initial phase of rapid accretion. Our results also suggest that violent relaxation plays a minor role in structuring CDM halos.
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Alexia Schulz



Joined: 03 Jun 2005
Posts: 2
Affiliation: Berkeley

PostPosted: September 02 2005  Reply with quote

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.
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Andrey Kravtsov



Joined: 23 Sep 2005
Posts: 2
Affiliation: University of Chicago

PostPosted: September 23 2005  Reply with quote

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 ρr−1 profile. However, I don't think this assumption is supported by results of cosmological simulations or results of controlled merger experiments
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