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M.A. Alvarez, P.R. Shapiro, H. Martel (Department of Astronomy, University of Texas at Austin)
Adaptive SPH and N-body simulations were carried out to study the collapse and evolution of dark matter haloes that result from the gravitational instability and fragmentation of cosmological pancakes. Such haloes resemble those formed by hierarchical clustering from realistic initial conditions in a CDM universe and, therefore, serve as a convenient test-bed model for studying halo dynamics. The halo density profile agrees with the fit to N-body results for CDM haloes by Navarro, Frenk, & White (NFW). The halo is in approximate virial 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, which we attribute to the cold, anisotropic initial conditions from which the haloes form.
Our test-bed model enables us to study the evolution of individual haloes as they grow. The mass of our haloes 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 as a result of finite mass supply. During the intermediate stage, the mass evolution resembles that of self-similar spherical infall, with M(a) proportional to the cosmic scale factor a. After the initial stage of collapse and virialization (a=acoll), the concentration parameter grows linearly with a, c(a)~4(a/acoll). The virial ratio 2T/|W| just after virialization is about 1.35, a value close to that of the N-body results for CDM haloes, as predicted by the truncated isothermal sphere model (TIS) 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 for N-body simulations of CDM analyzed by following the evolution of individual haloes. We therefore conclude that the fundamental properties of halo collapse, virialization, equilibrium structure, and evolution are generic to the formation of cosmological haloes by gravitational instability and are not limited to hierarchical collapse scenarios or even to Gaussian-random-noise initial conditions. This work was supported in part by grants NASA ATP NAG5-10825 and NAG5-10826 and Texas Advanced Research Program 3658-0624-1999. MAA is supported by a DOE Computational Science Graduate Fellowship.
The author(s) of this abstract have provided an email address for comments about the abstract: marcelo@astro.as.utexas.edu
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.