Hubble space telescope studies of the WLM galaxy. I. The age and metallicity of the globular cluster

We have obtained V and I images of the lone globular cluster that belongs t o 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 ha s 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 Gala xy. From the fit we also find that the distance modulus of the cluster is 2 4.73 +/- 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 f ormation of WLM, despite the parent galaxy's very small intrinsic mass and size.