The UV colours of high-redshift early-type galaxies: Evidence for recent star formation and stellar mass assembly over the last 8 billion years

We combine deep optical and NIR (UBVRIzJK) photometry from the Multiwavelength Survey by Yale-Chile (MUSYC) with redshifts from the COMBO-17 survey to perform a large-scale study of the rest-frame ultraviolet (UV) properties of 674 high-redshift (0.5 <z <1) early-type galaxies, drawn from the Extended Chandra Deep Field-South (E-CDFS). Galaxy morphologies are determined through visual inspection of Hubble Space Telescope (HST) images taken from the GEMS survey. We harness the sensitivity of the UV to young ( 0.5, implying that the UV is dominated by young stars, we find compelling evidence that early-types of all luminosities form stars over the lifetime of the Universe, although the bulk of their star formation is already complete at high redshift. This 'tail-end' of star formation is measurable and not negligible, with luminous [-23 <M(V) <-20.5] early-types potentially forming 10-15 per cent of their mass since z = 1, with less luminous early-types [M(V) > -20.5] potentially forming 30-60 per cent of their mass after z = 1. This, in turn, implies that intermediate-age stellar populations should be abundant in local early-type galaxies, as expected in hierarchical cosmology.