Local Group Dwarf Galaxies: Nature And Nurture
  We investigate the formation and evolution of dwarf galaxies in a high
resolution, hydrodynamical cosmological simulation of a Milky Way sized halo
and its environment. Our simulation includes gas cooling, star formation,
supernova feedback, metal enrichment and UV heating. In total, 90 satellites
and more than 400 isolated dwarf galaxies are formed in the simulation,
allowing a systematic study of the internal and environmental processes that
determine their evolution. We find that 95% of satellite galaxies are gas-free
at z=0, and identify three mechanisms for gas loss: supernova feedback, tidal
stripping, and photo-evaporation due to re-ionization. Gas-rich satellite
galaxies are only found with total masses above ~ 5x10^9 solar masses. In
contrast, for isolated dwarf galaxies, a total mass of ~ 10^9 solar masses
constitutes a sharp transition; less massive galaxies are predominantly
gas-free at z=0, more massive, isolated dwarf galaxies are often able to retain
their gas. In general, we find that the total mass of a dwarf galaxy is the
main factor which determines its star formation, metal enrichment, and its gas
content, but that stripping may explain the observed difference in gas content
between field dwarf galaxies and satellites with total masses close to 10^9
solar masses. We also find that a morphological transformation via tidal
stripping of infalling, luminous dwarf galaxies whose dark matter is less
concentrated than their stars, cannot explain the high total mass-light ratios
of the faint dwarf spheroidal galaxies.
