The Metallicity Distribution Function of Field Stars in M31's Bulge

We have used Hubble Space Telescope Wide Field Planetary Camera 2observations to construct a color-magnitude diagram (CMD) for the bulge of M31at a location ~1.6 kpc from the galaxy's center. Using scaled-solar abundancetheoretical red giant branches with a range of metallicities, we havetranslated the observed colors of the stars in the CMD to abundances andconstructed a metallicity distribution function (MDF) for this region. The MDFshows a peak at [M/H]~0 with a steep decline at higher metallicities and a moregradual tail to lower metallicities. This is similar in shape to the MDF of theMilky Way bulge but shifted to higher metallicities by ~0.1 dex. As is the casewith the Milky Way bulge MDF, a pure closed box model of chemical evolution,even with significant pre-enrichment, appears to be inconsistent with the M31bulge MDF. However, a scenario in which an initial infall of gas enriched thebulge to an abundance of [M/H] ~ -1.6 with subsequent evolution proceeding as aclosed box provides a better fit to the observed MDF. The similarity betweenthe MDF of the M31 bulge and that of the Milky Way stands in stark contrast tothe significant differences in the MDFs of their halo populations. Thissuggests that the bulk of the stars in the bulges of both galaxies were inplace before the accretion events that occurred in the halos could influencethem.Comment: 12 pages, 9 figures, accepted for publication in The Astronomical Journal, October 200