Journal Article

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

S. Kaviraj, S. Khochfar, K. Schawinski, S. K. Yi, E. Gawiser, J. Silk, S. N. Virani, C. N. Cardamone, P. G. Van Dokkum and C. M. Urry

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 388, issue 1, pages 67-79
Published in print July 2008 | ISSN: 0035-8711
Published online July 2008 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.13392.x
The UV colours of high-redshift early-type galaxies: evidence for recent star formation and stellar mass assembly over the last 8 billion years

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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.

Keywords: galaxies: elliptical and lenticular, cD; galaxies: evolution; galaxies: formation; galaxies: fundamental parameters

Journal Article.  11001 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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