An HST/ACS View of the Inhomogeneous Outer Halo of M31
  We present a high precision photometric view of the stellar populations in
the outer halo of M31, using data taken with the Hubble Space Telescope
Advanced Camera for Surveys (HST/ACS). We analyse the field populations
adjacent to 11 luminous globular clusters which sample the galactocentric
radial range 18 < R < 100 kpc and reach a photometric depth of ~2.5 magnitudes
below the horizontal branch (m_F814W ~27 mag). The colour-magnitude diagrams
(CMDs) are well populated out to ~60 kpc and exhibit relatively metal-rich red
giant branches, with the densest fields also showing evidence for prominent red
clumps. We use the Dartmouth isochrones to construct metallicity distribution
functions (MDFs) which confirm the presence of dominant populations with
<[Fe/H]> = -0.6 to -1.0 dex and considerable metallicity dispersions of 0.2 to
0.3 dex (assuming a 10 Gyr population and scaled-Solar abundances). The average
metallicity over the range 30 - 60 kpc is [Fe/H] = -0.8 +/- 0.14 dex, with no
evidence for a significant radial gradient. Metal-poor stars ([Fe/H] <= -1.3)
typically account for < 10-20 % of the population in each field, irrespective
of radius. Assuming our fields are unbiased probes of the dominant stellar
populations in these parts, we find that the M31 outer halo remains
considerably more metal-rich than that of the Milky Way out to at least 60 kpc.
