  We present new constraints on the star formation histories of six ultra-faint
dwarf galaxies: Bootes I, Canes Venatici II, Coma Berenices, Hercules, Leo IV,
and Ursa Major I. Our analysis employs a combination of high-precision
photometry obtained with the Advanced Camera for Surveys on the Hubble Space
Telescope, medium-resolution spectroscopy obtained with the DEep Imaging
Multi-Object Spectrograph on the W.M. Keck Observatory, and updated
Victoria-Regina isochrones tailored to the abundance patterns appropriate for
these galaxies. The data for five of these Milky Way satellites are best fit by
a star formation history where at least 75% of the stars formed by z~10 (13.3
Gyr ago). All of the galaxies are consistent with 80% of the stars forming by
z~6 (12.8 Gyr ago) and 100% of the stars forming by z~3 (11.6 Gyr ago). The
similarly ancient populations of these galaxies support the hypothesis that
star formation in the smallest dark matter sub-halos was suppressed by a global
outside influence, such as the reionization of the universe.
