The ACS LCID project. III. The star formation history of the Cetus dSph
  galaxy: a post-reionization fossil
  We use deep HST/ACS observations to calculate the star formation history
(SFH) of the Cetus dwarf spheroidal (dSph) galaxy. Our photometry reaches below
the oldest main sequence turn-offs, which allows us to estimate the age and
duration of the main episode of star formation in Cetus. This is well
approximated by a single episode that peaked roughly 12+/-0.5 Gyr ago and
lasted no longer than about 1.9+/-0.5 Gyr (FWHM). Our solution also suggests
that essentially no stars formed in Cetus during the past 8 Gyrs. This makes
Cetus' SFH comparable to that of the oldest Milky Way dSphs. Given the current
isolation of Cetus in the outer fringes of the Local Group, this implies that
Cetus is a clear outlier in the morphology-Galactocentric distance relation
that holds for the majority of Milky Way dwarf satellites. Our results also
show that Cetus continued forming stars through z ~ 1, long after the Universe
was reionized, and that there is no clear signature of the epoch of
reionization in Cetus' SFH. We discuss briefly the implications of these
results for dwarf galaxy evolution models. Finally, we present a comprehensive
account of the data reduction and analysis strategy adopted for all galaxies
targeted by the LCID (Local Cosmology from Isolated Dwarfs) project. We employ
two different photometry codes (DAOPHOT/ALLFRAME and DOLPHOT), three different
SFH reconstruction codes (IAC-pop/MinnIAC, MATCH, COLE), and two stellar
evolution libraries (BaSTI and Padova/Girardi), allowing for a detailed
assessment of the modeling and observational uncertainties.
