Dwarf Spheroidal Satellite Formation in a Reionized Local Group
June 06, 2013
Abstract: Dwarf spheroidal satellites in the Local Group exhibit a continuity of structural and chemical properties that extends to dwarf irregulars in the field to the more massive spheroidals in high-density environments. By connecting empirical constraints derived for star formation at low gas column densities and metallicities with a model for dark matter and baryonic mass assembly, we attempt a new look at how the dwarf spheroidals formed their stars. Star formation proceeded until the resident gas content, initially reduced from the cosmic average by the thermal pressure of the reionized intergalactic medium, was finally ram pressure stripped during the accretion onto the Milky Way or Andromeda. Satellites of differing luminosities seem to share similar most massive progenitor histories that reach thresholds for gas cooling by atomic line emission at epochs at which the Local Group should have been reionized, and so we hypothesize that was star formation under reionized conditions that defined their properties. This hypothesis leads us to reproduce the empirical luminosity-size and luminosity-metallicity relations. We speculate that the true fossils of the pre-reionization era should exhibit structure distinct from that of the dwarf spheroidals.