The ACS Nearby Galaxy Survey Treasury VIII. The Global Star Formation
  Histories of 60 Dwarf Galaxies in the Local Volume
  We present uniformly measured star formation histories (SFHs) of 60 nearby
(D~4Mpc) dwarf galaxies based on CMDs of resolved stellar populations from
images taken with HST as part of the ACS Nearby Galaxy Survey Treasury program
(ANGST). This volume-limited sample contains 12 dSph/dE, 5 dwarf spiral, 28 dI,
12 transition, and 3 tidal dwarf galaxies. From the best fit SFHs we find three
significant results: (1) the average dwarf galaxy formed >50% of its stars by
z~2 and 60% of its stars by z~1, regardless of current morphological type; (2)
the mean SFHs of dIs, dTrans, and dSphs are similar over most of cosmic time,
with the clearest differences appearing during the most recent 1 Gyr; and (3)
the mean values are inconsistent with simple SFH models, e.g., exponentially
declining SFRs. The mean SFHs are in general agreement with the cosmic SFH,
although we observe offsets near z~1 that could be evidence that low mass
systems experienced delayed star formation relative to more massive galaxies.
The sample shows a strong density-morphology relationship, such that the dSphs
are less isolated than dIs. We find that the transition from a gas-rich to
gas-poor galaxy cannot be solely due to internal mechanisms such as stellar
feedback, and instead is likely the result of external mechanisms, e.g., ram
pressure and tidal forces. The average transition dwarf is slightly less
isolated and less gas-rich than the typical dI. Further, the transition dwarfs
can be divided into two groups: interacting and gas-poor or isolated and
gas-rich, suggesting two possible evolutionary pathways.
