An HST/ACS view of the inhomogeneous outer halo of M31

Abstract

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. 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 mag below the horizontal branch (m F814W ∼ 27 mag). The colour-magnitude diagrams 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 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] = -0.80 ± 0.14 dex, with no evidence for a significant radial gradient. Metal-poor stars ([Fe/H] ≤ -1.3) typically account for ≲10-20 per cent 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.