Central Relaxation of Dark Matter Halos

Abstract

In this paper we reconsider a series expansion for a dark matter distribution function in the spherically symmetric anisotropic limit. We show here that the expansion may be renormalized so that the series does converge in time to an estimate of the steady state distribution function in the central regions. Subsequently, we use this distribution function to discuss the nature of the central equilibrium and by invoking stationarity of Boltzmann's H function as a measure of (thermodynamic) relaxation, we calculate the adiabatic variation in the local logarithmic slope of the mass density. Similarly, the pseudo- (phase-space) density variation with radius is calculated. These are compared to empirical fitting functions. There is general agreement on the inner part of the logarithmic slope of the density and of the inner profile of the pseudodensity power law, but coincident continuity with the outer power laws is not yet achieved. Finally, some suggestions are made regarding the actual microphysics acting during the nonequilibrium approach to relaxation. In particular a cascade regime is identified.