Coarse-graining the distribution function of cold dark matter

Abstract

The density profiles established in the self-similar dynamical phase of dark matter haloes recollapse are all close to isothermal. This is steeper than the predictions of some n-body simulations for the central regions of the halo, which are in turn steeper than the profiles observed in the centre of some galaxies; particularly dwarfs and low surface brightness galaxies. The outer regions of galaxies both as observed and as simulated have density profiles steeper than the self-similar profile. Nevertheless there appears to be an intermediate region in most galaxies in which the inverse square behaviour is a good description. The outer deviations can be explained plausibly in terms of the transition from a self-gravitating extended halo to a Keplerian flow onto a dominant central mass (the isothermal distribution can not be complete), but the inner deviations are more problematic. Rather than attack this question directly, we use in this paper a novel coarse-graining technique combined with a shell code to establish both the distribution function associated with the self-similar density profile and the nature of the possible deviations in the central regions. In spherical symmetry we find that both in the case of purely radial orbits and in the case of orbits with non-zero angular momentum the self-similar density profile should flatten progressively near the centre of the system. The NFW limit of -1 seems possible. In a section aimed at demonstrating our technique for a spherically symmetric steady state, we argue that a Gaussian distribution function is the best approximation near the centre of the system.