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

Abstract

We derive ages, metallicities and [alpha/Fe] ratios from the integrated spectra of 23 globular clusters in M31, by employing multivariate fits to two stellar population models. In parallel we analyze spectra of 21 Galactic globular clusters in order to facilitate a differential analysis. We find that the M31 globular clusters separate into three distinct components in age and metallicity. 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 not identified in the Galactic globular cluster sample. The majority of globular clusters in both samples appear to be enhanced in alpha-elements, the degree of enhancement being model-dependent. The intermediate age GCs appear to be the most enhanced, with [alpha/Fe]~0.4. These clusters are clearly depressed in CN with 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 the stellar 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 not leading to erroneous age estimates in our analysis. The intermediate age clusters have generally higher velocities than the bulk of the M31 cluster population. Spatially, three of these clusters are projected onto the bulge region, the remaining three are distributed at large radii. We discuss these objects within the context of the build-up of the M31 halo, and suggest that these clusters possibly originated in a gas-rich dwarf galaxy, which may or may not be presently observable in M31.