HST/ACS Direct Ages of the Dwarf Elliptical Galaxies NGC 147 and NGC 185
  We present the deepest optical photometry for any dwarf elliptical (dE)
galaxy based on Hubble Space Telescope ACS observations of the Local Group dE
galaxies NGC 147 and NGC 185. The resulting F606W and F814W color-magnitude
diagrams are the first to reach below the main sequence turnoff in a dE galaxy,
allowing us to determine full star formation histories in these systems. The
ACS fields are located ~1.5 effective radii from the galaxy center to avoid
photometric crowding. While our ACS pointings in both dEs show unambiguous
evidence for old and intermediate age stars, the mean age in NGC 147 is ~ 4 Gyr
younger as compared to NGC 185. In NGC 147, only 40% of stars were in place
12.5 Gyrs ago (z~5), with the bulk of the remaining stellar population forming
between 5 to 7 Gyr. In contrast, 70% of stars were formed in NGC 185 field more
than12.5 Gyr ago with the majority of the remaining population forming between
8 to 10 Gyr. Star formation ceased in both ACS fields at least 3 Gyr ago.
Previous observations in the central regions of NGC 185 show evidence for star
formation as recent as 100 Myr ago and a strong metallicity gradient with
radius. We suggest that the orbit of NGC 185 has a larger pericenter as
compared to NGC 147, allowing it to preserve radial gradients and maintain a
small central reservoir of recycled gas. We interpret the inferred differences
in star formation histories to imply an earlier infall time into the M31
environment for NGC 185 as compared to NGC 147.
