The Star Formation Histories of Local Group Dwarf Galaxies I. Hubble Space Telescope / Wide Field Planetary Camera 2 Observations
Daniel R. Weisz, Andrew E. Dolphin, Evan D. Skillman, Jon Holtzman,, Karoline M. Gilbert, Julianne J. Dalcanton, and Benjamin F. Williams

TL;DR
This study analyzes star formation histories of 40 Local Group dwarf galaxies using Hubble data, revealing patterns in their evolution, environment effects, and early star formation efficiencies, with implications for galaxy formation models.
Contribution
It provides uniformly measured SFHs for 40 dwarf galaxies, demonstrating the importance of the oldest MSTO for early epoch constraints and comparing different galaxy types.
Findings
Dwarf spheroidals have exponentially declining SFHs with τ ~ 5 Gyr.
Lower luminosity dSphs have less extended SFHs.
Dwarf irregulars and other types show early exponential decline and later constant star formation.
Abstract
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 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 ( 3-4 Gyr) for lookback ages 10-12…
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