The Ages and Abundances of a Large Sample of M87 Globular Clusters

A subset of 150 globular clusters in M87 has been selected on the basis ofS/N ratio for abundance and age determinations from the sample of Paper I.Indices measuring the strength of the strongest spectral features weredetermined for the M87 GCs and from new data for twelve galactic GCs. Combiningthe new and existing data for the galactic GCs and comparing the $(U-R)$ colorsand the line indices gives qualitative indications for the ages and abundancesof the GCs. Quantitative results are obtained by applying the Worthey (1994)models for the integrated light of stellar systems of a single age, calibratedby observations of galactic GCs, to deduce abundances and ages for the objectsin our sample. We find that the M87 GCs span a wide range in metallicity, from very metalpoor to somewhat above solar metallicity. The mean [Fe/H] of -0.95 dex ishigher than that of the galactic GC system, and there is a metal rich tail thatreaches to higher [Fe/H] than one finds among the galactic GCs. The meanmetallicity of the M87 GC system is about a factor of four lower than that ofthe M87 stellar halo at a fixed projected radius $R$. The metallicity inferredfrom the X-ray studies is similar to that of the M87 stellar halo, not to thatof GCs. We infer the relative abundances of Na, Mg, and Fe in the M87 GCs fromthe strength of their spectral features. The behavior of these elements betweenthe metal rich and metal poor M87 GCs is similar to that shown by the galacticGCs and by halo stars in the Galaxy. The pattern of chemical evolution in thesedisparate old stellar systems is indistinguishable. We obtain a median age forthe M87 GC system of 13 Gyr, similar to that of the galactic GCs, with a smalldispersion about this value.Comment: 56 pages with included postscript figures; added derived M87 GC metallicities to Table 2, a statistical analysis of possible bimodality, an appendix on the metallicity calibration of U-R and the Washington system, and other smaller changes. Accepted for publication in ApJ. (See paper for complete version of the Abstract.