Friday, June 21, 2013

1306.4703 (Milos Milosavljevic et al.)

Dwarf Spheroidal Satellite Formation in a Reionized Local Group    [PDF]

Milos Milosavljevic, Volker Bromm
The population of dwarf spheroidal satellite galaxies in the Local Group has emerged as a powerful probe of dark matter clustering on small spatial scales and of cosmic reionization. The dwarf spheroidals are also interesting in view of the continuity of structural and chemical properties they share with each other, with dwarf irregular galaxies in the field, and with the more massive spheroidal galaxies in high-density environments. By connecting empirical constraints derived for star formation at low gas column densities and metallicities in the local universe with a model for dark matter and baryonic mass assembly, we provide an analytical description of how the dwarf spheroidals acquired their stellar content. Their progenitors formed stars until their gas content, initially reduced from the cosmic average by the thermal pressure of the reionized intergalactic medium, is finally ram pressure stripped during the progenitors' accretion onto the host galaxy. Dwarf spheroidal satellites of differing luminosities seem to share very similar most massive progenitor histories that reach thresholds for gas cooling by atomic line emission at epochs at which the Lagrangian volume of the Local Group should have been reionized. We hypothesize that it is the star formation in a reionized universe, rather than preceding reionization, that defines their properties. This model provides an explanation for the "common mass scale" relation of Strigari et al. (2008) and reproduces the empirical luminosity-size and luminosity-metallicity relations if assuming that star formation was quiescent and confined to the cold atomic phase stochastically condensing in partially rotationally supported HI disks.
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