The Observed properties of Dark Matter on small spatial scales

There has long been evidence that low-mass galaxies are systematically larger in radius, of lower central stellar mass density, and of lower central phase-space density, than are star clusters of the same luminosity. The larger radius, at a comparable value of central velocity dispersion, implies a larger mass at similar luminosity, and hence significant dark matter, in dwarf galaxies, compared to no dark matter in star clusters. We present a synthesis of recent photometric and kinematic data for several of the most dark-matter dominated galaxies. There is a bimodal distribution in half-light radii, with stable star clusters always being smaller than ∼ 30pc, while stable galaxies are always larger than ∼ 120pc. 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 100pc, and always has a maximum central mass density with 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 about 0.1M⊙ pc −3 (about 5GeV/c cm). All dSphs have velocity Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK; [email protected] The Johns Hopkins University, Department of Physics & Astronomy, 3900 N. Charles Street, Baltimore, MD 21218, USA Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA Department of Physics and Astronomy, UCLA, 430 Portola Plaza, Los Angeles, CA 90095-1547, USA Astronomical Insitute of the University of Basel, Department of Physics and Astronomy, Venusstr. 7, CH-4102 Binningen, Switzerland Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK. Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 1214, D-69120 Heidelberg, Germany.