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 similar to 30 pc, while stable galaxies are always larger than similar to 120 pc. We extend the previously known observational relationships and interpret them in terms of a more fundamental pair of intrinsic properties of darkmatter itself: dark matter forms coredmass 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 (circle dot) 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 similar to 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 similar to 0.1M circle dot pc - 3 for the favored cored dark mass distributions ( where it is similar to the mean value), or similar to 60M (circle dot) 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.