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 \la~ { } { \mathrm { R } } \la 100 ~ { } { \mathrm { kpc } } and reach a photometric depth of \sim 2.5 magnitudes below the horizontal branch ( m _ { F 814 W } \sim 27 mag ) .
The colour-magnitude diagrams ( CMDs ) are well populated out to \sim 60 ~ { } { \mathrm { 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 < [ \mbox { Fe / H } ] > \approx - 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 \pm 0.14 dex , with no evidence for a significant radial gradient .
Metal-poor stars ( [ Fe/H ] \leq - 1.3 ) typically account for \la 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 .