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, Sukyoung Yi, E. Gawiser, J. Silk, S. N. Virani, C. N. Cardamone, P. G. van Dokkum, C. M. Urry

Research output: Contribution to journalArticle

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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.

Original languageEnglish
Pages (from-to)67-79
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Volume388
Issue number1
DOIs
Publication statusPublished - 2008 Dec 1

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stellar mass
star formation
assembly
galaxies
color
stars
Chile
cosmology
photometry
universe
luminosity
life (durability)
young

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Kaviraj, S. ; Khochfar, S. ; Schawinski, K. ; Yi, Sukyoung ; Gawiser, E. ; Silk, J. ; Virani, S. N. ; Cardamone, C. N. ; van Dokkum, P. G. ; Urry, C. M. / The UV colours of high-redshift early-type galaxies : Evidence for recent star formation and stellar mass assembly over the last 8 billion years. In: Monthly Notices of the Royal Astronomical Society. 2008 ; Vol. 388, No. 1. pp. 67-79.
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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. / Kaviraj, S.; Khochfar, S.; Schawinski, K.; Yi, Sukyoung; Gawiser, E.; Silk, J.; Virani, S. N.; Cardamone, C. N.; van Dokkum, P. G.; Urry, C. M.

In: Monthly Notices of the Royal Astronomical Society, Vol. 388, No. 1, 01.12.2008, p. 67-79.

Research output: Contribution to journalArticle

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AU - Silk, J.

AU - Virani, S. N.

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