| Abstract
| - 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 (<1-Gyr old) stars to quantify the recent star formation history of early-type galaxies across a range of luminosities [−23.5 < M(V) < −18]. Comparisons to simple stellar populations forming at high redshift indicate that ∼1.1 per cent of early-types in this sample are consistent with purely passive ageing since z= 2 - this value drops to ∼0.24 per cent and ∼0.15 per cent for z= 3 and 5, respectively. Parametrizing the recent star formation (RSF) in terms of the mass fraction of stars less than a Gyr old, we find that the early-type population as a whole shows a typical RSF between 5 and 13 per cent in the redshift range 0.5 < z< 1. Early-types on the broad UV ‘red sequence’ show RSF values less than 5 per cent, while the reddest early-types (which are also the most luminous) are virtually quiescent with RSF values of ∼1 per cent. In contrast to their low-redshift (z< 0.1) counterparts, the high-redshift early-types in this sample show a pronounced bimodality in the rest-frame UV-optical colour, with a minor but significant peak centred on the blue cloud. Furthermore, star formation in the most active early-types is a factor of 2 greater at z∼ 0.7 than in the local universe. Given that evolved sources of UV flux (e.g. horizontal branch stars) should be absent at z> 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.
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