A rogues gallery of Andromeda's dwarf galaxies I. A predominance of red
  horizontal branches
  We present homogeneous, sub-horizontal branch photometry of twenty dwarf
spheroidal satellite galaxies of M31 observed with the Hubble Space Telescope.
Combining our new data for sixteen systems with archival data in the same
filters for another four, we show that Andromeda dwarf spheroidal galaxies
favor strikingly red horizontal branches or red clumps down to ~10^{4.2} Lsun
(M_V ~ -5.8). The age-sensitivity of horizontal branch stars implies that a
large fraction of the M31 dwarf galaxies have extended star formation histories
(SFHs), and appear inconsistent with early star formation episodes that were
rapidly shutdown. Systems fainter than ~10^{5.5} Lsun show the widest range in
the ratios and morphologies of red and blue horizontal branches, indicative of
both complex SFHs and a diversity in quenching timescales and/or mechanisms,
which is qualitatively different from what is currently known for faint Milky
Way (MW) satellites of comparable luminosities. Our findings bolster similar
conclusions from recent deeper data for a handful of M31 dwarf galaxies. We
discuss several sources for diversity of our data such as varying halo masses,
patchy reionization, mergers/accretion, and the environmental influence of M31
and the Milky Way on the early evolution of their satellite populations. A
detailed comparison between the histories of M31 and MW satellites would shed
signifiant insight into the processes that drive the evolution of low-mass
galaxies. Such a study will require imaging that reaches the oldest main
sequence turnoffs for a significant number of M31 companions.
