A Model for the Formation and Evolution of Cosmological Halos

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 from realistic initial conditions in a CDM universe and, therefore, serve as a convenient testbed 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 (1997; 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 involving rapid mass assembly, an intermediate stage of continuous infall, and a final stage in which infall tapers off as a result of finite mass supply. In the intermediate stage, halo mass grows at the rate expected for self-similar spherical infall, with M(a) ∝ a. After the end of initial collapse at (a ≡ a0), the concentration parameter grows linearly with the cosmic scale factor a, c(a) ≃ 4(a/a0). The virial ratio 2T/|W | just after virialization is about 1.35, a value close to that of the N -body results for CDM halos, as predicted by the truncated isothermal sphere model (TIS) (Shapiro, Iliev, & Raga 1999) 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, as the mass infall rate declines. This mass accretion history and evolution of concentration parameter are very similar to those reported recently in N -body simulations of CDM analyzed by following the evolution of individual halos. We therefore conclude that the fundamental properties of halo collapse, virialization, structure, and evolution are generic to the formation of cosmological halos by gravitational instability and are not limited to hierarchical collapse