Photometry and the Metallicity Distribution of the Outer Halo of M31

We have conducted a wide-field CCD-mosaic study of the resolved red giant branch stars of M31, in a field located 20 kpc from the nucleus along the southeast minor axis. In our (I, V-I) color-magnitude diagram, red giant branch (RGB) stars in the top three magnitudes of the M31 halo are strongly present. We use photometry of a more distant control field to subtract field contamination and then to derive the "cleaned" luminosity function and metallicity distribution for this outer-halo region of M31. From the color distribution of the foreground Milky Way halo stars, we find a reddening E(V-I) = 0.10 ± 0.02 for this field, and from the luminosity of the RGB tip we determine a distance modulus (m - M)0 = 24.47 ± 0.12 (=783 ± 43 kpc). The metallicity distribution function (MDF) is derived from interpolation within an extensive new grid of RGB models (Vandenberg et al.). We find that the MDF is dominated by a moderately high-metallicity population ([m/H] ∼ -0.5) that has previously been found in more interior M31 halo/bulge fields, and is very much more metal-rich than the [m/H] ∼ -1.5 level which characterizes the Milky Way halo. In addition, a significant (∼30%–40%, depending on AGB star contribution) metal-poor population is also present. To first order, the total shape of the MDF resembles that predicted by a simple, single-component model of chemical evolution starting from primordial gas with an effective yield y = 0.0055. It strongly resembles the MDF recently found by Harris et al. for the outer halo of the giant elliptical NGC 5128, though NGC 5128 has an even lower fraction of low-metallicity stars. Intriguingly, in both NGC 5128 and M31, the metallicity distribution of the globular clusters in M31 does not match the halo stars, in the sense that the clusters are far more heavily weighted to metal-poor objects. We suggest similarities in the formation and early evolution of massive, spheroidal stellar systems.