The Observed Properties of Dark Matter on Small Spatial Scales

We present a synthesis of recent photometric and kinematic data for several of the most dark matter dominated galaxies, the dwarf spheroidal Galactic satellites, and compare them to star clusters. There is a bimodal distribution in half-light radii, with stable star clusters always being smaller than ~30 pc, while stable galaxies are always larger than ~120 pc. We extend the previously known observational relationships and interpret them in terms of a more fundamental pair of intrinsic properties of dark matter itself: dark matter forms cored mass distributions, with a core scale length of greater than about 100 pc, and always has a maximum central mass density within a narrow range. The dark matter in dSph galaxies appears to be clustered such that there is a mean volume mass density within the stellar distribution which has the very low value of less than about 0.1 M_☉ pc^-3 (about 5 GeV/c² cm^-3). All dSph's have velocity dispersions at the edge of their light distributions equivalent to circular velocities of ~15 km s^-1. The maximum central dark matter density derived is model dependent but is likely to have a characteristic value (averaged over a volume of radius 10 pc) of ~0.1 M_☉ pc^-3 for the favored cored dark mass distributions (where it is similar to the mean value), or ~60 M_☉ pc^-3 (about 2 TeV/c² cm^-3) if the dark matter density distribution is cusped. Galaxies are embedded in dark matter halos with these properties; smaller systems containing dark matter are not observed. These values provide new information about the nature of the dominant form of dark matter.