The structure of dark matter halos in hierarchical clustering theories

Abstract

During hierarchical clustering, smaller masses generally collapse earlier than larger masses and so are denser on the average. The core of a small mass halo could be dense enough to resist disruption and survive undigested, when it is incorporated into a bigger object. We explore the possibility that a nested sequence of undigested cores in the center of the halo, which have survived the hierarchical, inhomogeneous collapse to form larger and larger objects, determines the halo structure in the inner regions. For a flat universe with $P(k) \propto k^n$, scaling arguments then suggest that the core density profile is, $\rho \propto r^{-\alpha}$ with $\alpha = (9+3n)/(5+n)$. But whether such behaviour obtains depends on detailed dynamics. We first examine the dynamics using a fluid approach to the self-similar collapse solutions for the dark matter phase space density, including the effect of velocity dispersions. We highlight the importance of tangential velocity dispersions to obtain density profiles shallower than $1/r^2$ in the core regions. If tangential velocity dispersions in the core are constrained to be less than the radial dispersion, a cuspy core density profile shallower than 1/r cannot obtain, in self-similar collapse. We then briefly look at the profiles of the outer halos in low density cosmological models where the total halo mass is convergent. Finally, we analyze a suite of dark halo density and velocity dispersion profiles obtained in cosmological N-body simulations of models with n= 0, -1 and -2. We find that the core-density profiles of dark halos, show considerable scatter in their properties, but nevertheless do appear to reflect a memory of the initial power spectrum, with steeper initial spectra producing flatter core profiles. (Abridged)Comment: 31 pages, 7 figures, submitted to Ap