A Model for the Formation and Evolution of Cosmological Haloes

Adaptive smoothed particle hydrodynamics (ASPH) and N-body simulations were carried out to study the collapse and evolution of dark matter haloes that result from the gravitational instability of cosmological pancakes. Such haloes resemble those formed by hierarchical clustering from realistic initial conditions in a cold dark matter (CDM) universe, serving as a test-bed model for studying halo dynamics. The halo density profile agrees with the fit to N-body results by Navarro, Frenk, & White (NFW). The halo is in approximate equilibrium and roughly isothermal; it satisfies the Jeans equation to good accuracy. Our haloes are somewhat less isotropic than typically found in simulations of CDM, attributable to the cold, anisotropic initial conditions from which the haloes form.

Our model enables us to study the growth of individual haloes. The mass evolves in three stages: an initial collapse involving rapid mass assembly, followed by an intermediate stage of continuous infall, ending in a third stage in which infall tapers off. In the intermediate stage, the mass evolution resembles that of self-similar spherical infall, with M(a) proportional to the scale factor a. After the initial stage of collapse (a=acoll ), the concentration grows linearly with a, c(a)~4(a/acoll ). The virial ratio 2T/|W| just after virialization is ~1.35, close to that of the N-body results for CDM haloes, as predicted by the truncated isothermal sphere model (TIS) and consistent with 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 similar to those reported for N-body simulations of CDM analyzed by following the evolution of individual haloes. We therefore conclude that the fundamental properties of halo collapse and evolution are generic to the formation of cosmological haloes by gravitational instability and are not limited to hierarchical collapse scenarios or even Gaussian-random-noise initial conditions.

Abstract Author(s): Marcelo A. Alvarez, Paul R. Shapiro, and Hugo Martel