Newborn spheroids at high redshift: When and how did the dominant, old stars in today's massive galaxies form?

S. Kaviraj, J. Silk, R.S. Ellis, S. Cohen, R.A. Windhorst, S. Peirani, R.W. O'Connell, B.C. Whitmore, R.E. Ryan Jr., N.P. Hathi, M.A. Dopita, J.A. Frogel, A. Dekel

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We study ~330 massive (M > 10M), newborn spheroidal galaxies (SGs) around the epoch of peak star formation (1 <z <3) to explore the high-redshift origin of SGs and gain insight into when and how the old stellar populations that dominate today's Universe formed. The sample is drawn from the Hubble Space Telescope (HST)/WFC3 Early-Release Science programme, which provides deep 10-filter (0.2-1.7 μm) HST imaging over one-third of the GOODS-South field. We find that the star formation episodes that built our SGs likely peaked in the redshift range 2 <z <5 (with a median of z ~ 3) and have decay time-scales shorter than ~1.5Gyr. Starburst time-scales and ages show no trend with stellar mass in the range 10 <M <10 M. However, the time-scales show increased scatter towards lower values ( 10M, and an age trend becomes evident in this mass regime: SGs with M > 10M are ~2 Gyr older than their counterparts with M <10M. Nevertheless, a smooth downsizing trend with galaxy mass is not observed, and the large scatter in starburst ages indicates that SGs are not a particularly coeval population. Around half of the blue SGs appear not to drive their star formation via major mergers, and those that have experienced a recent major merger show only modest enhancements (~40 per cent) in their specific star formation rates. Our empirical study indicates that processes other than major mergers (e.g. violent disc instability driven by cold streams and/or minor mergers) likely play a dominant role in building SGs, and creating a significant fraction of the old stellar populations that dominate today's Universe.
Original languageEnglish
Pages (from-to)925-934
Number of pages10
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date1 Nov 2012
Publication statusPublished - 11 Jan 2013


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