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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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Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH 2 Yonsei University, Centre for Space Astrophysics, Seoul 120749, Korea 3 Yale Center for Astronomy and Astrophysics, Yale University, New Haven, CT 06520-8121, USA

KEYWORDS

galaxies: elliptical and lenticular, cD • galaxies: evolution • galaxies: formation • galaxies: fundamental parameters

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.

Accepted 2008 April 28. Received 2008 April 21; in original form 2007 September 5

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Evidence for a Population of High-Redshift Submillimeter Galaxies from Interferometric Imaging

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ABSTRACT. We have used the Submillimeter Array to image a flux-limited sample of seven submillimeter galaxies, selected by the AzTEC camera on the JCMT at 1.1 mm, in the COSMOS field at 890 μ m with ~2'' resolution. All of the sources—two radio-bright and five radio-dim—are detected as single point sources at high significance (>6 σ), with positions accurate to ~0.2'' that enable counterpart identification at other wavelengths observed with similarly high angular resolution. All seven have IRAC counterparts, but only two have secure counterparts in deep HST ACS imaging. As compared to the two radio-bright sources in the sample, and those in previous studies, the five radio-dim sources in the sample (1) have systematically higher submillimeter-to-radio flux ratios, (2) have lower IRAC 3.6-8.0 μ m fluxes, and (3) are not detected at 24 μ m . These properties, combined with size constraints at 890 μ m (θ lesssim 1.2''), suggest that the radio-dim submillimeter galaxies represent a population of very dusty starbursts, with physical scales similar to local ultraluminous infrared galaxies, with an average redshift higher than radio-bright sources.

Subject headings: cosmology: observations; galaxies: evolution; galaxies: formation; galaxies: high-redshift; galaxies: starburst; submillimeter

Print publication: Issue 2 (2007 December 20)
Received 2007 June 7, accepted for publication 2007 August 6