The Ages and Abundances of a Sample of Globular Clusters in M49 (NGC 4472)

Abstract

We present a study of the metallicity and age of the globular clusters system of M49 (NGC 4472), the most luminous galaxy in the Virgo Cluster. We measure Lick indices for 47 globular clusters (GCs) in M49 from LRIS/Keck spectra and establish their metallicity parameters qualitatively in comparison to the Galactic GCs and to published data for 150 M87 GCs. We then compare our measurements with the predictions of models for the integrated light of old single-burst stellar systems by Worthey and by Thomas, Maraston, & Bender. We find that the metallicity of the M49 GC system spans the range from [Fe/H]Z=-2.0 to +0.4 dex. We show that the metallicity and age parameters for these two GC systems are basically identical, except that the M49 GCs reach slightly higher metallicities than do those of M87. We find that the GCs of both of these giant elliptical galaxies are α-enhanced by a factor of about 2 above the solar value, as is also true of the Galactic GCs. Thus the most metal rich GCs in M49 reach [Z/H]~+0.8 dex, comparable to that of M49 itself. While adoption of the α-enhanced models of Thomas et al. has eliminated most of the previous discrepancies with observations, the most metal rich M49 GCs have Na D lines that are still considerably stronger than those predicted by any model, and there are still issues involving the metallicity scale of these models. We find that in the mean, the M49 GCs are at least 10 Gyr old. However, the grids of models we used differ in how they treat the horizontal branch, and this perceptibly affects the predicted Hβ index. Hence our current incomplete understanding of the role of the horizontal branch limits our ability to derive a more detailed distribution for the ages of the GCs in M49 and in M87. Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration.