The Chemical Properties of Milky Way and M31 Globular Clusters. I. A Comparative Study

A comparative analysis is performed between high-quality integrated spectraof 30 globular clusters in M31, 20 Milky Way clusters, and a sample of fieldand cluster elliptical galaxies. We find that the Lick CN indices in the M31and Galactic clusters are enhanced relative to the bulges of the Milky Way,M31, and elliptical spheroids. Although not seen in the Lick CN indices, thenear-UV cyanogen feature (3883 A) is strongly enhanced in M31 clustesr withrespect to the Galactic globulars at metallicities, --1.5<[Fe/H]<--0.3. Carbonshows signs of varying amongst these two groups. For [Fe/H]>--0.8, we observeno siginificant differences in the Hdelta, Hgamma, or Hbeta indices between theM31 and Galactic globulars. The sample of ellipticals lies offset from the lociof all the globulars in the Cyanogen--[MgFe], and Balmer--[MgFe] planes. Six ofthe M31 cluster spectra appear young, and are projected onto the M31 disk.Population synthesis models suggest that these are metal-rich clusters withages 100--800 Myr, metallicities --0.20 < [Fe/H] <0.35, and masses 0.7-7.0x10^4 Msun. Two other young clusters are Hubble V in NGC 205, and an older(~3 Gyr) cluster ~7 kpc away from the plane of the disk. The six clustersprojected onto the disk rotate in a similar fashion to the HI gas in M31, andthree clusters exhibit thin disk kinematics (Morrison et al.). Dynamical massesand structural parameters are required for these objects to determine whetherthey are massive open clusters or globular clusters. If the latter, ourfindings suggest globular clusters may trace the build up of galaxy disks. Ineither case, we conclude that these clusters are part of a young, metal-richdisk cluster system in M31, possibly as young as 1 Gyr old.Comment: 52 pages, 14 figures, 8 tables, minor revisions in response to referee, conclusions remain the same. Scheduled to appear in the October 2004 issue of The Astronomical Journa