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Published January 11, 2013 | Published
Journal Article Open

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

Abstract

We study ∼330 massive (M* > 109.5 M_⊙), 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 ∼1.5 Gyr. Starburst time-scales and ages show no trend with stellar mass in the range 10^(9.5) < M* < 10^(10.5) M_⊙. However, the time-scales show increased scatter towards lower values (<0.3 Gyr) for M* > 10^(10.5) M_⊙, and an age trend becomes evident in this mass regime: SGs with M* > 10^(11.5) M_⊙ are ∼2 Gyr older than their counterparts with M* < 10^(10.5) M_⊙. 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.

Additional Information

© 2012 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2012 September 18. Received 2012 September 8; in original form 2012 June 6. First published online: November 1, 2012. We are grateful to the referee Scott Trager for many constructive comments that helped improve the original manuscript. Daniel Thomas, Pieter van Dokkum, Ignacio Ferreras, Mariska Kriek, Claudia Maraston, Simona Mei and Ewan Cameron are thanked for comments and related discussions. SK is grateful for the generous hospitality of the California Institute of Technology, where most of this work was completed. SK also acknowledges fellowships from Imperial College London, the Royal Commission for the Exhibition of 1851 and Worcester College, Oxford. This paper is based on ERS observations made by theWFC3 Scientific Oversight Committee. We are grateful to the Director of the Space Telescope Science Institute for awarding Director's Discretionary time and deeply indebted to the brave astronauts of STS-125 for rejuvenating HST. Support for HST programme 11359 was provided by NASA through grant GO-11359 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. RAW also acknowledges support from NASA JWST Interdisciplinary Scientist grant NAG5-12460 from GSFC. The work of AD has been partly supported by the ISF grant 6/08 by GIF grant G-1052-104.7/2009, DIP grant STE1869/1-1.GE625/15-1 and NSF grant AST-1010033.

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