Hubble Space TelescopeStudies of the WLM Galaxy. I. The Age and Metallicity of the Globular Cluster

Abstract

We have obtained V and I images of the lone globular cluster that belongs to the dwarf Local Group irregular galaxy known as WLM. The color-magnitude diagram of the cluster shows that it is a normal old globular cluster with a well-defined giant branch reaching to M_V=-2.5, a horizontal branch at M_V=+0.5, and a subgiant branch extending to our photometry limit of M_V=+2.0. A best fit to theoretical isochrones indicates that this cluster has a metallicity of [Fe/H]=-1.52+/-0.08 and an age of 14.8+/-0.6 Gyr, thus indicating that it is similar to normal old halo globulars in our Galaxy. From the fit we also find that the distance modulus of the cluster is 24.73+/-0.07 and the extinction is A_V=0.07+/-0.06, both values that agree within the errors with data obtained for the galaxy itself by others. We conclude that this normal massive cluster was able to form during the formation of WLM, despite the parent galaxy's very small intrinsic mass and size.

Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.