A Genuine Intermediate-Age Globular Cluster in M33

Abstract

We present deep integrated-light spectroscopy of nine M33 globular clusters taken with the Hectospec instrument at the MMT Observatory. Based on our spectroscopy and previous deep color-magnitude diagrams obtained with HST WFPC2, we present evidence for the presence of a genuine intermediate-age globular cluster in M33. The analysis of Lick line indices indicates that all globular clusters are metal-poor ([Z/H]<~-1.0) and that cluster M33-C38 is ~5-8 Gyr younger than the rest of the sample M33 star clusters. We find no evidence for a population of blue horizontal-branch stars in the CMD of M33-C38, which rules out the possibility of an artificially young spectroscopic age due to the presence of hot stars. We infer an initial mass of (0.8-1.2)×10⁵ M_solar for M33-C38, which implies that intermediate-age clusters with masses similar to those of Galactic globular clusters were able to form and survive in M33, although it is not yet clear with which dynamical component of M33-thin disk, thick disk, halo-the cluster is associated.

Based on observations obtained with the Hectospec instrument at the MMT Observatory. The MMT Observatory is a joint venture of the Smithsonian Institute and the University of Arizona. Also based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.