M31's Heavy-Element Distribution and Outer Disk

Hubble Space Telescope imaging of 11 fields in M31 were reduced to color-magnitude diagrams. The fields were chosen to sample all galactocentric radii to 50 kpc (-9 disk scale lengths, or >99.9% of the total light enclosed). The colors of the red giants at each pointing map to an abundance distribution with errors of order 0.1 dex in [M/H]. The abundance distributions are all about the same width but show a mild gradient that flattens outside s20 kpc. These distributions were weighted and summed with the aid of a new surface brightness profile fit to obtain an abundance distribution representative of the entirety of M31. Since we expect M31 to have retained most of its congenital gas and subsequently accreted material, and since the present-day gas mass fraction is around 2%, it must be a system near chemical maturity. This "observed closed box" is found to suffer from a lack of metal-poor stars and metal-rich stars relative to the simplest closed-box model in the same way as the solar neighborhood. Models modified to include inhomogeneous chemical enrichment, variable yield, or infall all fit to within the uncertainties. As noted elsewhere, stars in the outer regions of M31 are a factor of 10 more metal-rich than the Milky Way halo, 10 times more metal-rich than the dwarf spheroidals cospatial with it, and more metal-rich than most of the globular clusters at the same galactocentric radius. Difficulties of interpretation are greatly eased if we posit that the M31 disk dominates over the halo at all radii out to 50 kpc. In fact, scaling from current density models of the Milky Way, one should not expect to see halo stars dominating over disk stars until beyond our 50 kpc limit. A corollary conclusion is that most published studies of the M31 "halo" are actually studies of its disk.