Metal abundances for a large sample of globular clusters in M87

Abstract

We present deep integrated Washington CCD photometry of the extremely populous globular cluster system of NGC 4486 (M87) in Virgo. The metallicities of 407 of the best candidates with T_1_ < 22 have been estimated from the (C - T_1_) color. These values are internally precise to ~0.15 dex and accurate to ~0.25 dex. The mean metallicity of the globular clusters is [Fe/H] = -0.86 dex, with a formal standard error of the mean of 0.03 dex but a more realistic uncertainty of about 0.2 dex. This value is higher than that of the corresponding means of the globular clusters in the Galaxy and M 31, but similar to those of the globular clusters in NGC 1399 and NGC 5128, as expected from the relative parent galaxy luminosities. ApproximatelY 10% of the clusters exceed solar abundance, similar to the case for the globular clusters in NGC 1399 and NGC 5128. As found by Couture et al. [ApJS, 73,671(1990)], there is a large metallicity dispersion at any radius, with a σ ~ 0.65 dex. However, in contrast to their result and that of previous small-field CCD studies, the globular clusters display a marked gradient in both mean (M- T_1_) and (C - T_1_) color with galactocentric distance. The mean metallicity decreases by some 0.6 dex over the radial extent of the cluster sample. This result is consistent with the early photographic study by Strom et al. [ApJ, 245,416 (1981)]. The metallicity distribution function is remarkably similar to that of NGC 1399 found by Ostrov et al. [AJ, 105,1762(1993)] using the same technique. The two central cluster ellipticals show significant peaks at very similar metallicities, indicating similar formation and enrichment histories. We also find that this structure in the metallicity distribution function persists to radii where the halo light is dominated by the cD envelope.