An Ancient Metal-Poor Population in M32, and Halo Satellite Accretion in
  M31, Identified by RR Lyrae Stars
  We present time-series photometry of two fields near M32 using archival
observations from ACS/WFC onboard HST. One field is centered about 2 arcmin
from M32 while the other is located 15 arcmin to the southeast of M31. We
identify a total of 1139 RR Lyrae variables of which 821 are ab-type and 318
are c-type. In the field near M32, we find a radial gradient in the density of
RR Lyraes relative to the center of M32. This gradient is consistent with the
surface brightness profile of M32 suggesting that a significant number of the
RR Lyraes in this region belong to M32. This provides further confirmation that
M32 contains an ancient stellar population formed around the same time as the
oldest population in M31 and the Milky Way. The RR Lyrae stars in M32 exhibit a
mean metal abundance of [Fe/H] ~ -1.42 +/- 0.02, which is ~15 times lower than
the metal abundance of the overall M32 stellar population. Moreover, the
abundance of RR Lyrae stars normalized to the luminosity of M32 in the field
analyzed further indicates that the ancient metal-poor population in M32
represents only a very minor component of this galaxy, consistent with the 1%
to 4.5% in mass inferred from the CMD analysis of Monachesi et al. In the other
field, we find unprecedented evidence for two populations of RR Lyraes in M31
as shown by two distinct sequences among the ab-type variables in the Bailey
Diagram. When interpreted in terms of metal abundance, one population exhibits
a peak at [Fe/H] ~ -1.3 and the other is at [Fe/H] ~ -1.9. One possible
interpretation of this result is that the more metal-rich population represents
the dominant M31 halo, while the metal-poorer group could be a disrupted dwarf
satellite galaxy orbiting M31. If true, this represents a further indication
that the formation of the M31 spheroid has been significantly influenced by the
merger and accretion of dwarf galaxy satellites. [abridged]
