16459

We propose to obtain a color-magnitude diagram (CMD) of the globular cluster (GC) EXT8 in M31. With a metallicity of CORCHETE_OPENFe-HCORCHETE_CLOSE=-2.9 comma EXT8 is by far the most metal-poor GC yet identified comma and as such it provides us with unique information on CMD morphology at this low metallicity. At the same time comma it challenges theories for the formation of GCs in the context of hierarchical galaxy formation. We will image the cluster with WFC3 in the F300X comma F606W comma and F814W filters and obtain a CMD that reaches below the horizontal branch. The observations will thus provide the first empirical insight into horizontal branch morphology in a GC near CORCHETE_OPENFe-HCORCHETE_CLOSE=-3. Photometry of stars on the red giant branch will provide an independent verification of the spectroscopic metallicity determination and will constrain any metallicity spread present in the cluster (that might be suggestive of an origin as a stripped nucleus).