The Chemical Properties of Milky Way and M31 Globular Clusters. II. Stellar Population Model Predictions

We derive ages, metallicities and [alpha/Fe] ratios from the integratedspectra of 23 globular clusters in M31, by employing multivariate fits to twostellar population models. In parallel we analyze spectra of 21 Galacticglobular clusters in order to facilitate a differential analysis. We find thatthe M31 globular clusters separate into three distinct components in age andmetallicity. We identify an old, metal-poor group (7 clusters), an old,metal-rich group (10 clusters) and an intermediate age (3-6 Gyr),intermediate-metallicity ([Z/H]~-1) group (6 clusters). This third group is notidentified in the Galactic globular cluster sample. The majority of globularclusters in both samples appear to be enhanced in alpha-elements, the degree ofenhancement being model-dependent. The intermediate age GCs appear to be themost enhanced, with [alpha/Fe]~0.4. These clusters are clearly depressed in CNwith respect to the models and the bulk of the M31 and Milky Way sample.Compared to the bulge of M31, M32 and NGC 205, these clusters most resemble thestellar populations in NGC 205 in terms of age, metallicity and CN abundance.We infer horizontal branch morphologies for the M31 clusters using the Rose(1984) Ca II index, and demonstrate that blue horizontal branches are notleading to erroneous age estimates in our analysis. The intermediate ageclusters have generally higher velocities than the bulk of the M31 clusterpopulation. Spatially, three of these clusters are projected onto the bulgeregion, the remaining three are distributed at large radii. We discuss theseobjects within the context of the build-up of the M31 halo, and suggest thatthese clusters possibly originated in a gas-rich dwarf galaxy, which may or maynot be presently observable in M31.Comment: 19 figures, 1 table, accepted for publication in the Astronomical Journa