Dark matter phase space densities

The low velocity part of a kinetic equilibrium dark matter distribution has higher phase space density and is more easily incorporated in formation of a low mass galaxy than the high velocity part. For relativistically decoupling fermions (bosons), this explains one (two) orders of magnitude of the observed trend, that phase space densities in dark matter halo cores are highest in the smallest systems, and loosens constraints on particle masses significantly. For non-relativistic decoupling and/or finite chemical potentials even larger effects may occur. It is therefore premature to dismiss dissipationless particle distributions as dark matter on the basis of phase space arguments.