Authors:Smecker-Hane, T. A., Marsteller, B., Bullock, J. (Univ. of California, Irvine), Cole, A. (Univ. of Tasmania), Gallagher, J. (Univ. of Wisconsin, Madison)

We report on results a new deep imaging survey obtained with the Hubble Space Telesscope's Advance Camera for Surveys (ACS) and the Wide Field Planetary Camera 2 (WFPC2) that shows the Leo I dwarf spheroidal galaxy (dSph) has a significantly large population of ancient ($> 10$ Gyr old) stars as well as the previously known large component of intermediate-aged stars. Our new imaging is much deeper than a previous WFPC2 study, which allows us to unabiguously identify the main sequence turnoff of the ancient population and constrain the star formation rate at early times. We determine the galaxy's star formation rate as a function of time from the observed density of stars in the color-magnitude diagram using Padova stellar evolutionary models, and we discuss how our results may help explain the flat age-metallicity relationship we previously inferred from spectroscopy and age-determination of individual red giant stars. This galaxy, an isolated dwarf located at a distance of 270 kpc, lies well outside the dark matter halo of the Milky Way, and its evolution demonstrates that star formation in the Local Group began at roughly the same time shortly after the Big Bang over a wide range of galactic masses.