WHY BARYONS MATTER: THE KINEMATICS OF DWARF SPHEROIDAL SATELLITES

Abstract

We use high resolution cosmological simulations of Milky Way-mass galaxies\nthat include both baryons and dark matter to show that baryonic physics\n(energetic feedback from supernovae and subsequent tidal stripping)\nsignificantly reduces the dark matter mass in the central regions of luminous\nsatellite galaxies. The reduced central masses of the simulated satellites\nreproduce the observed internal dynamics of Milky Way and M31 satellites as a\nfunction of luminosity. We use these realistic satellites to update predictions\nfor the observed velocity and luminosity functions of satellites around Milky\nWay-mass galaxies when baryonic effects are accounted for. We also predict that\nfield dwarf galaxies in the same luminosity range as the Milky Way classical\nsatellites should not exhibit velocities as low as the satellites, since the\nfield dwarfs do not experience tidal stripping. Additionally, the early\nformation times of the satellites compared to field galaxies at the same\nluminosity may be apparent in the star formation histories of the two\npopulations. Including baryonic physics in Cold Dark Matter models naturally\nexplains the observed low dark matter densities in the Milky Way's dwarf\nspheroidal population. Our simulations therefore resolve the tension between\nkinematics predicted in Cold Dark Matter theory and observations of satellites,\nwithout invoking alternative forms of dark matter.\n