A Model for the Formation and Evolution of Cosmological Halos

Abstract

Adaptive SPH and N-body simulations were carried out to study the collapse and evolution of dark matter halos that result from the gravitational instability and fragmentation of cosmological pancakes. Such halos resemble those formed by hierarchical clustering in a CDM universe and serve as a convenient test-bed model for studying halo dynamics. Our halos are in approximate virial equilibrium and roughly isothermal, as in CDM simulations. Their density profiles agree quite well with the fit to N-body results for CDM halos by Navarro, Frenk, & White (NFW). This test-bed model enables us to study the evolution of individual halos. The masses of our halos evolve in three stages: an initial collapse, continuous infall, and a final stage in which infall tapers off as a result of finite mass supply. In the continuous infall stage, halo mass grows at the rate expected for self-similar spherical infall, with M(a) proportional to the scale factor a. After the end of initial collapse at a=a_0, the concentration parameter grows linearly with a, c(a)~4a/a_0. The virial ratio 2T/|W| just after virialization is about 1.35, as predicted by the truncated isothermal sphere model and consistent with the value expected for a virialized halo in which mass infall contributes an effective surface pressure. Thereafter, the virial ratio evolves towards the value expected for an isolated halo, 2T/|W|~1. This mass accretion history and evolution of concentration parameter are very similar to those reported recently in N-body simulations of CDM. We therefore conclude that the fundamental properties of halo formation and evolution are generic to the formation of cosmological halos by gravitational instability and are not limited to hierarchical collapse scenarios or even to Gaussian-random-noise initial conditions.