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; similar to 1530 angstrom) and near-UV (NUV; similar to 2310 angstrom) 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 similar to 90 per cent more constrained and metallicity estimates that are similar to 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.