VLT spectroscopy of globular clusters in the Sombrero galaxy

Abstract

We have obtained intermediate-resolution VLT spectroscopy of 75 globular cluster candidates around the Sa galaxy M104 (NGC4594). Fifty-seven candidates out to ~ 40 kpc in the halo of the galaxy were confirmed to be bona-fide globular clusters, 27 of which are new. A first analysis of the velocities provides only marginal evidence for rotation of the cluster system. From Hbeta line strengths, almost all of the clusters in our sample have ages that are consistent, within the errors, with Milky Way globular clusters. Only a few clusters may be 1-2 Gyr old, and bulge and halo clusters appear coeval. The absorption line indices follow the correlations established for the Milky Way clusters. Metallicities are derived based upon new empirical calibrations with Galactic globular clusters taking into account the non-linear behavior of some indices (e.g., Mg2). Our sample of globular clusters in NGC4594 spans a metallicity range of -2.13 < [Fe/H] < +0.26 dex, and the median metallicity of the system is [Fe/H] = -0.85. Thus, our data provide evidence that some of the clusters have super-solar metallicity. Overall, the abundance distribution of the cluster system is consistent with a bimodal distribution with peaks at [Fe/H] ~- 1.7 and -0.7. However, the radial change in the metallicity distribution of clusters may not be straightforwardly explained by a varying mixture of two sub-populations of red and blue clusters.