New light on the formation and evolution of M 31 and its globular cluster system

¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA e-mail: tpuzia@stsci.edu
² Department of Astronomy & Astrophysics, University of Toronto, Toronto, ON, M5S 3H8, Canada e-mail: perrett@astro.utoronto.ca
³ Department of Physics, Queen's University, Kingston, ON, K7L 3N6, Canada e-mail: tjb@astro.queensu.ca

Received: 6 August 2004
Accepted: 13 January 2005

Abstract

We present spectroscopic ages, metallicities, and [ α/Fe]  ratios for 70 globular clusters in M 31 that were derived from Lick line-index measurements. A new interpolation technique of age-metallicity and α/Fe-diagnostic grids is used to account for changes in index strength as a response to abundance-ratio variations, in particular for all of the Balmer-line Lick indices. In addition to a population of old (>10 Gyr) globular clusters with a wide range of metallicities, from about -2.0 dex to solar values, we find evidence for a population of intermediate-age globular clusters with ages between ~5 and 8 Gyr and a mean metallicity [Z/H] ≈ -0.6. We also confirm the presence of young M 31 globular clusters that were recently identified by Beasley et al. (2004, AJ, 128, 1623), which have ages 1 Gyr and relatively high metallicities around -0.4 dex. The M 31 globular cluster system has a clearly super-solar mean [ α/Fe]  = 0.14 ± 0.04 dex. Intermediate-age and young objects show roughly solar abundance ratios. We find evidence for an age-[ α/Fe]  relation in the sense that younger clusters have smaller mean [ α/Fe]  ratios. From a comparison of indices, mostly sensitive to carbon and/or nitrogen abundance, with SSP model predictions for nitrogen-enhanced stellar populations, we find a dichotomy in nitrogen enhancement between young and old M 31 globular clusters. The indices of objects older than 5 Gyr are consistent with a factor of three or higher in nitrogen enhancement compared to their younger counterparts. Using kinematical data from Morrison et al. (2004, ApJ, 603, 87) we find that the globular cluster sub-population with halo kinematics is old (9 Gyr), has a bimodal metallicity distribution, and super-solar [ α/Fe] . Disk globular clusters have a wider range of ages, are on average more metal-rich, and have a slightly smaller mean [ α/Fe] ratio. A cross-correlation of structural parameters for M 31 globular clusters with spectroscopically derived ages, metallicities, and [ α/Fe]  ratios shows a correlation between half-light/tidal radius and metallicity, which is most likely due to the correlation of half-light/tidal radius and galactocentric distance. We compare our results for M 31 globular clusters with those obtained with the same technique for globular clusters in the Milky Way, Large Magellanic Cloud, M 81, and other spiral galaxies in the Sculptor group. Finally, we compare the globular cluster systems of the two Local Group spirals, M 31 and Milky Way, with their integrated bulge light.