The chemical properties of Milky Way and M31 globular clusters. I. A comparative study

A comparative analysis is performed between high-quality integrated spectral indices of 30 M31 globular clusters, 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 and coworkers. Although not particularly evident in the Lick CN indices, the near-UV cyanogen feature (A3883) is strongly enhanced with respect to the Galactic globular clusters at metallicities -1.5 < [Fe/H] < -0.3. Carbon shows signs of varying among 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 globular clusters, in contrast to previous studies. The elliptical galaxy sample lies offset from the loci of the globular clusters in 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 × 104 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 H i gas in M31, and three clusters exhibit thin disk kinematics, according to Morrison and coworkers. 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 buildup 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.