The Chemical Properties of Milky Way and M31 Globular Clusters. I. A Comparative Study (original) (raw)

2004, The Astronomical Journal

A comparative analysis is performed between high-quality integrated spectral indices of 30 globular clusters in M31, 20 Milky Way globular clusters, and a sample of field and cluster elliptical galaxies. We find that the Lick CN indices in the M31 and Galactic clusters are enhanced relative to the bulges of the Milky Way, M31, and elliptical spheroids, in agreement with Burstein et al. Although not particularly evident in the Lick CN indices, the near-UV cyanogen feature (λ3883) is strongly enhanced with respect to the Galactic globulars at metallicities, -1.5<[Fe/H]<-0.3. Carbon shows signs of varying amongst these two groups. For [Fe/H]>-0.8, we observe no systematic differences in the Hδ, Hγ, or Hβ indices between the M31 and Galactic globulars, in contrast to previous studies. The elliptical galaxy sample lies offset from the loci of the globular clusters in the both the Cyanogen-[MgFe], and Balmer-line-[MgFe] planes. Six of the M31 clusters appear young, and are projected onto the M31 disk. Population synthesis models suggest that these are metal-rich clusters with ages 100-800 Myr, metallicities -0.20 ≤ [Fe/H] ≤ 0.35, and masses 0.7 ∼ 7.0 × 10 4 M ⊙ . Two other young clusters are Hubble V in NGC 205, observed as a template, and an older (∼3 Gyr) cluster some 7 kpc away from the plane of the disk. The six clusters projected onto the disk show signs of rotation similar to the HI gas in M31, and three clusters exhibit thin disk kinematics (Morrison et al.). Dynamical mass estimates and detailed structural parameters are required for these objects to determine whether they are massive open clusters or globular clusters. If they are the latter, our findings suggest globular clusters may trace the build up of galaxy disks. In either case, we conclude that these clusters are part of a young, metal-rich disk cluster system in M31, possibly as young as 1 Gyr old.