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We use deep HST/ACS observations to calculate the star formation history(SFH) of the Cetus dwarf spheroidal (dSph) galaxy. Our photometry reaches belowthe oldest main sequence turn-offs, which allows us to estimate the age andduration of the main episode of star formation in Cetus. This is wellapproximated by a single episode that peaked roughly 12+/-0.5 Gyr ago andlasted no longer than about 1.9+/-0.5 Gyr (FWHM). Our solution also suggeststhat essentially no stars formed in Cetus during the past 8 Gyrs. This makesCetus' SFH comparable to that of the oldest Milky Way dSphs. Given the currentisolation of Cetus in the outer fringes of the Local Group, this implies thatCetus is a clear outlier in the morphology-Galactocentric distance relationthat holds for the majority of Milky Way dwarf satellites. Our results alsoshow that Cetus continued forming stars through z ~ 1, long after the Universewas reionized, and that there is no clear signature of the epoch ofreionization in Cetus' SFH. We discuss briefly the implications of theseresults for dwarf galaxy evolution models. Finally, we present a comprehensiveaccount of the data reduction and analysis strategy adopted for all galaxiestargeted by the LCID (Local Cosmology from Isolated Dwarfs) project. We employtwo different photometry codes (DAOPHOT/ALLFRAME and DOLPHOT), three differentSFH reconstruction codes (IAC-pop/MinnIAC, MATCH, COLE), and two stellarevolution libraries (BaSTI and Padova/Girardi), allowing for a detailedassessment of the modeling and observational uncertainties.

The ACS LCID project. III. The star formation history of the Cetus dSph galaxy: a post-reionization fossil