BR photometry of the halo of M31

A deep (R_lim_ ~ 25) color-magnitude diagram of a 1'x 2' field in the halo of M31 9 kpc from the nucleus has been constructed, reaching just to the level of the horizontal branch. The color spread of the red giant branch is significantly larger than expected from observational uncertainties, and is (as concluded by previous works) likely due to a spread in [Fe/H] of the halo stars. Comparison of the red giant branch with color-calibrated isochrones shows that its mean metallicity is high (<[Fe/H]>~ -0.6), with an intrinsic dispersion of ~0.2-0.3 dex. Modeling of the CMD with artificial star experiments suggests that the M31 halo has only a very weak blue HB component, with most HB stars even redder than in 47 Tucanae. These results further demonstrate that the halo stars of M31 in this field are more metal-rich than the M31 globular clusters, as has been observed for many large galaxies. However, this situation contrasts strongly with the Milky Way, where the halo clusters and field stars have fairly similar metallicity distribution functions (MDFs); in addition, it suggests that M31 underwent considerably more pre-enrichment (by 1 dex in (Fe/H]) before the main epoch of halo formation.