Mass growth and density profiles of dark matter halos in hierarchical clustering

Abstract

We develop a model for the growth of dark matter halos and use it to study their evolved density profiles. In this model, halos are spherical and form by quiescent accretion of matter in clumps, called satellites. The halo mass as a function of redshift is given by the mass of the most massive progenitor, and is determined from Monte-Carlo realizations of the merger-history tree. Inside the halo, satellites move under the action of the gravitational force of the halo and a dynamical friction drag force. The associated equation of motion is solved numerically. The energy lost to dynamical friction is transferred to the halo in the form of kinetic energy. As they sink into the halo, satellites continually lose matter as a result of tidal stripping. The stripped matter moves inside the halo free of dynamical friction. The evolved density profiles are steeper than those obtained by assuming that, once they have been accreted onto the parent halo, satellites remain at a fixed distance from the halo center. We find that the final density profile depends mainly on the rate of infall of matter onto the halo. This, in turn, depends on the initial fluctuation field as well as on cosmology. For mass scales where the effective spectral index of the initial density field is less than -1, the model predicts a profile which can only approximately be matched by the one parameter family of curves suggested by Navarro, Frenk and White (1997). For scale-free power-spectra with initial slope $n$, the density profile within about 1% of the virial radius is $ρ\propto r^{-β}$, with $3(3+n)/(5+n)\leβ\le 3(3+n)/(4+n)$.