Dwarf Spheroidal Satellite Formation in a Reionized Local Group
  Dwarf spheroidal satellite galaxies have emerged a powerful probe of
small-scale dark matter clustering and of cosmic reionization. They exhibit
structural and chemical continuity with dwarf irregular galaxies in the field
and with spheroidal galaxies in high-density environments. By combining
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 the gas content, initially reduced from the cosmic average by the thermal
pressure of the reionized intergalactic medium, was finally ram pressure
stripped during the progenitors' accretion on to 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 dwarf spheroidals formed the
bulk of their stars in partially rotationally supported HI disks in a reionized
universe. This model provides an explanation for the "common mass scale"
relation and reproduces the empirical luminosity-size and
luminosity-metallicity relations. Explosive feedback phenomena, such as
outflows driven by the concerted action of supernovae, need not have been
significant in the dwarf spheroidals' formation. We further speculate that the
true pre-reionization fossils should exhibit a structure distinct from that of
the dwarf spheroidals, e.g., in the form of dense isolated or nuclear star
clusters.
