Better age estimation using ultraviolet–optical colours: breaking the age–metallicity degeneracy

Abstract 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.