Comparing M31 and Milky Way Satellites: The Extended Star Formation
  Histories of Andromeda II and Andromeda XVI
  We present the first comparison between the lifetime star formation histories
(SFHs) of M31 and Milky Way (MW) satellites. Using the Advanced Camera for
Surveys aboard the Hubble Space Telescope, we obtained deep optical imaging of
Andromeda II (M$_{V} = -$12.0; log(M$_{\star}$/M$_{\odot}$) $\sim$ 6.7) and
Andromeda XVI (M$_{V} = -$7.5; log(M$_{\star}$/M$_{\odot}$) $\sim$ 4.9)
yielding color-magnitude diagrams that extend at least 1 magnitude below the
oldest main sequence turnoff, and are similar in quality to those available for
the MW companions. And II and And XVI show strikingly similar SFHs: both formed
50-70% of their total stellar mass between 12.5 and 5 Gyr ago (z$\sim$5-0.5)
and both were abruptly quenched $\sim$ 5 Gyr ago (z$\sim$0.5). The predominance
of intermediate age populations in And XVI makes it qualitatively different
from faint companions of the MW and clearly not a pre-reionization fossil.
Neither And II nor And XVI appears to have a clear analog among MW companions,
and the degree of similarity in the SFHs of And II and And XVI is not seen
among comparably faint-luminous pairs of MW satellites. These findings provide
hints that satellite galaxy evolution may vary substantially among hosts of
similar stellar mass. Although comparably deep observations of more M31
satellites are needed to further explore this hypothesis, our results underline
the need for caution when interpreting satellite galaxies of an individual
system in a broader cosmological context.
