The persistence of universal halo profiles

We have performed simple simulations which suggest that the phase-space structure of halos identified in cosmological calculations is invariant under the dynamics induced by sinking substructure satellites. This contention is confirmed using a two-component Fokker-Planck formulation, representing the dynamics of a smooth background and a system of massive clumps. It is shown that, to very high accuracy, when the clumps sink in under the action of dynamical friction, the background expands so as to leave the total distribution unchanged. This holds for the inner and intermediate regions of isotropic systems in dynamical equilibrium, and is valid for any mass spectrum of substructure, because the governing equation is linear in their mass-weighted phase-space distribution. If the clumps are considered solid, the process whereby background particles are driven out of low-energy states takes the form of an exponential instability, with a characteristic timescale on the order of the dynamical friction time, on which develops a low-energy cutoff in the phase-space distribution of these lighter particles and a constant-density core in their spatial distribution. This could correspond to a situation in which the clumps are made of dense baryonic material. We also considered the case when stripping is strong enough for a low-energy cutoff to develop in the clump distribution ( as in the situation with dissipationless substructure). The results of this paper suggest that halo profiles similar to those found in dissipationless cosmological simulations are approximately invariant under the interaction induced by the presence of substructure, a necessary condition for the observed universality.'' In addition, the total profile, including baryons, should also be invariant, provided that the latter are initially in the form of dense clumps, whose distribution initially follows that of dark matter.