Astronomy and Astrophysics, volume 434, 909-934 (2005/5-2)
New light on the formation and evolution of M 31 and its globular cluster system.
PUZIA T.H., PERRETT K.M. and BRIDGES T.J.
Abstract (from CDS):
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 (>10Gyr) 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. (2004AJ....128.1623B
), which have ages ≲1Gyr and relatively high metallicities around -0.4dex. The M 31 globular cluster system has a clearly super-solar mean [α/Fe]=0.14±0.04dex. 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. (2004ApJ...603...87M
) we find that the globular cluster sub-population with halo kinematics is old (>9Gyr), 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.
galaxies: star clusters - galaxies: formation - galaxies: evolution - galaxies: Local Group - Galaxy: globular clusters: general - galaxies: general
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