NATURE AND NURTURE IN DARK MATTER HALOS

Cosmological simulations consistently predict specific properties of dark matter halos, but these have not yet led to a physical understanding that is generally accepted. This is especially true for the central regions of these structures. Recently, two major themes have emerged. In one, the dark matter halo is primarily a result of the sequential accretion of the primordial structure (i.e., "nature"), while in the other, dynamical relaxation (i.e., "nurture") dominates at least in the central regions. Some relaxation is however required in either mechanism. In this paper, we accept the recently established scale-free substructure of halos as an essential part of both mechanisms. Consequently, a simple model for the central relaxation, based on a self-similar cascade of tidal interactions, is contrasted with a model based on the accretion of an adiabatically self-similar, primordial structure. We conclude that a weak form of this relaxation is present in the simulations, but that it is normally described as the radial orbit instability.