The Star Formation Histories of Local Group Dwarf Galaxies I. Hubble
  Space Telescope / Wide Field Planetary Camera 2 Observations
  We present uniformly measured star formation histories (SFHs) of 40 Local
Group dwarf galaxies based on color-magnitude diagram (CMD) analysis from
archival Hubble Space Telescope imaging. We demonstrate that accurate SFHs can
be recovered from CMDs that do not reach the oldest main sequence turn-off
(MSTO), but emphasize that the oldest MSTO is critical for precisely
constraining the earliest epochs of star formation. We find that: (1) the
average lifetime SFHs of dwarf spheroidals (dSphs) can be approximated by an
exponentially declining SFH with $\tau$ $\sim$ 5 Gyr; (2) lower luminosity
dSphs are less likely to have extended SFHs than more luminous dSphs; (3) the
average SFHs of dwarf irregulars (dIrrs), transition dwarfs (dTrans), and dwarf
ellipticals (dEs) can be approximated by the combination of an exponentially
declining SFH ($\tau$ $\sim$ 3-4 Gyr) for lookback ages $>$ 10-12 Gyr ago and a
constant SFH thereafter; (4) the observed fraction of stellar mass formed prior
to z=2 ranges considerably (80\% for galaxies with M $<$ 10$^5$ M$_{\odot}$ to
30\% for galaxies with M$>$10$^7$ M$_{\odot}$) and is largely explained by
environment; (5) the distinction between "ultra-faint" and "classical" dSphs is
arbitrary; (6) LG dIrrs formed a significantly higher fraction of stellar mass
prior to z=2 than the SDSS galaxies from Leiter 2012 and the SFHs from the
abundance matching models of Behroozi et al. 2013. This may indicate higher
than expected star-formation efficiencies at early times in low mass galaxies.
Finally, we provide all the SFHs in tabulated electronic format for use by the
community.
