  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.
