The Chemical Properties of Milky Way and Andromeda Globular Clusters: I. A Comparative Study

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Abstract
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 Burstein et al. Although not particularly evident in the Lick CN indices, the near-UV cyanogen feature (3850 A) is strongly enhanced with respect to the Galactic GCs 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 Hdelta, Hgamma, or Hbeta indices between the M31 and Galactic clusters, 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. Eight 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 0.7~7.0x10^4 Mo, compatible with young globular clusters. The remaining two 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.). We conclude that these clusters may be part of a young, metal-rich disk GC system in M31, possibly as young as 1 Gyr old. Our findings suggest globular clusters may trace the build up of galaxy disks.

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