Comparing the Quenching Times of Faint M31 and Milky Way Satellite
  Galaxies
  We present the star formation histories (SFHs) of 20 faint M31 satellites
($-12 \lesssim M_V \lesssim -6$) that were measured by modeling sub-horizontal
branch (HB) depth color-magnitude diagrams constructed from Hubble Space
Telescope (HST) imaging. Reinforcing previous results, we find that virtually
all galaxies quenched between 3 and 9 Gyr ago, independent of luminosity, with
a notable concentration $3-6$ Gyr ago. This is in contrast to the Milky Way
(MW) satellites, which are generally either faint with ancient quenching times
or luminous with recent ($<3$ Gyr) quenching times. We suggest that systematic
differences in the quenching times of M31 and MW satellites may be a reflection
of the varying accretion histories of M31 and the MW. This result implies that
the formation histories of low-mass satellites may not be broadly
representative of low-mass galaxies in general. Among the M31 satellite
population we identify two distinct groups based on their SFHs: one with
exponentially declining SFHs ($\tau \sim 2$ Gyr) and one with rising SFHs with
abrupt quenching. We speculate how these two groups could be related to
scenarios for a recent major merger involving M31. The Cycle 27 HST Treasury
survey of M31 satellites will provide well-constrained ancient SFHs to go along
with the quenching times we measure here. The discovery and characterization of
M31 satellites with $M_V \gtrsim -6$ would help quantify the relative
contributions of reionization and environment to quenching of the lowest-mass
satellites.
