We derive ages , metallicities and abundance ratios ( [ \alpha /Fe ] ) from the integrated spectra of 23 globular clusters in M31 , by employing multivariate fits to two different stellar population models .
We also perform a parallel analysis on 21 Galactic globular clusters as a consistency check , and in order to facilitate a differential analysis .
Our analysis shows 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 ] \sim –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 , but the degree of enhancement is rather model-dependent .
The intermediate age GCs appear to be the most enhanced , with [ \alpha /Fe ] \sim 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 .
We discuss and reject as unlikely the hypothesis that these objects are in fact foreground stars contaminating the optical catalogs .
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 .