The Formation History of the Ultra-Faint Dwarf Galaxies
  We present early results from a Hubble Space Telescope survey of the
ultra-faint dwarf galaxies. These Milky Way satellites were discovered in the
Sloan Digital Sky Survey, and appear to be an extension of the classical dwarf
spheroidals to low luminosities, offering a new front in the efforts to
understand the missing satellite problem. Because they are the least luminous,
most dark matter dominated, and least chemically evolved galaxies known, the
ultra-faint dwarfs are the best candidate fossils from the early universe. The
primary goal of the survey is to measure the star-formation histories of these
galaxies and discern any synchronization due to the reionization of the
universe. We find that the six galaxies of our survey have very similar
star-formation histories, and that each is dominated by stars older than 12
Gyr.
