Bibcode
DOI
Beasley, Michael A.; Brodie, Jean P.; Strader, Jay; Forbes, Duncan A.; Proctor, Robert N.; Barmby, Pauline; Huchra, John P.
Bibliographical reference
The Astronomical Journal, Volume 129, Issue 3, pp. 1412-1427.
Advertised on:
3
2005
Citations
52
Refereed citations
49
Description
We derive ages, metallicities, and abundance ratios ([α/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 (seven clusters), an old, metal-rich group (10
clusters), and an intermediate-age (3-6 Gyr), intermediate-metallicity
([Z/H]~-1) group (six clusters). This third group is not identified in
the Galactic globular cluster sample. We also see evidence that the old,
metal-rich Galactic globular clusters are 1-2 Gyr older than their
counterparts in M31. The majority of globular clusters in both samples
appear to be enhanced in α-elements, but the degree of enhancement
is rather model-dependent. The intermediate-age globular clusters appear
to be the most enhanced, with [α/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 with 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 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,
and 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.