THE STAR FORMATION HISTORIES OF LOCAL GROUP DWARF GALAXIES. I.<i>HUBBLE SPACE TELESCOPE</i>/WIDE FIELD PLANETARY CAMERA 2 OBSERVATIONS

Abstract

We present uniformly measured star formation histories (SFHs) of 40 Local\nGroup dwarf galaxies based on color-magnitude diagram (CMD) analysis from\narchival Hubble Space Telescope imaging. We demonstrate that accurate SFHs can\nbe recovered from CMDs that do not reach the oldest main sequence turn-off\n(MSTO), but emphasize that the oldest MSTO is critical for precisely\nconstraining the earliest epochs of star formation. We find that: (1) the\naverage lifetime SFHs of dwarf spheroidals (dSphs) can be approximated by an\nexponentially declining SFH with $\\tau$ $\\sim$ 5 Gyr; (2) lower luminosity\ndSphs are less likely to have extended SFHs than more luminous dSphs; (3) the\naverage SFHs of dwarf irregulars (dIrrs), transition dwarfs (dTrans), and dwarf\nellipticals (dEs) can be approximated by the combination of an exponentially\ndeclining SFH ($\\tau$ $\\sim$ 3-4 Gyr) for lookback ages $&gt;$ 10-12 Gyr ago and a\nconstant SFH thereafter; (4) the observed fraction of stellar mass formed prior\nto z=2 ranges considerably (80\\% for galaxies with M $&lt;$ 10$^5$ M$_{\\odot}$ to\n30\\% for galaxies with M$&gt;$10$^7$ M$_{\\odot}$) and is largely explained by\nenvironment; (5) the distinction between "ultra-faint" and "classical" dSphs is\narbitrary; (6) LG dIrrs formed a significantly higher fraction of stellar mass\nprior to z=2 than the SDSS galaxies from Leiter 2012 and the SFHs from the\nabundance matching models of Behroozi et al. 2013. This may indicate higher\nthan expected star-formation efficiencies at early times in low mass galaxies.\nFinally, we provide all the SFHs in tabulated electronic format for use by the\ncommunity.\n