Redefining the Missing Satellites Problem

Abstract

Numerical simulations of Milky-Way size Cold Dark Matter (CDM) halos predict\na steeply rising mass function of small dark matter subhalos and a substructure\ncount that greatly outnumbers the observed satellites of the Milky Way. Several\nproposed explanations exist, but detailed comparison between theory and\nobservation in terms of the maximum circular velocity (Vmax) of the subhalos is\nhampered by the fact that Vmax for satellite halos is poorly constrained. We\npresent comprehensive mass models for the well-known Milky Way dwarf\nsatellites, and derive likelihood functions to show that their masses within\n0.6 kpc (M_0.6) are strongly constrained by the present data. We show that the\nM_0.6 mass function of luminous satellite halos is flat between ~ 10^7 and 10^8\nM_\\odot. We use the ``Via Lactea'' N-body simulation to show that the M_0.6\nmass function of CDM subhalos is steeply rising over this range. We rule out\nthe hypothesis that the 11 well-known satellites of the Milky Way are hosted by\nthe 11 most massive subhalos. We show that models where the brightest\nsatellites correspond to the earliest forming subhalos or the most massive\naccreted objects both reproduce the observed mass function. A similar analysis\nwith the newly-discovered dwarf satellites will further test these scenarios\nand provide powerful constraints on the CDM small-scale power spectrum and warm\ndark matter models.\n