Better age estimation using ultraviolet-optical colours: Breaking the age-metallicity degeneracy

We demonstrate that the combination of GALEX ultraviolet (UV) photometry in the far-UV (FUV; ~1530 Å) and near-UV (NUV; ~2310 Å) passbands with optical photometry in the standard U, B, V, R, I filters can efficiently break the age-metallicity degeneracy. We estimate well-constrained ages, metallicities and their associated errors for 42 globular clusters (GCs) in M31, and show that the full set of FUV, NUV, U, B, V, R, I photometry produces age estimates that are ~90 per cent more constrained and metallicity estimates that are ~60 per cent more constrained than those produced by using optical filters alone. The quality of the age constraints is comparable or marginally better than those achieved using a large number of spectroscopic indices.