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The UV continua and inferred stellar populations of galaxies at z ≃ 7–9 revealed by the Hubble Ultra-Deep Field 2012 campaign

Dunlop, J. S. and Rogers, A. B. and McLure, R. J. and Ellis, R. S. and Robertson, B. E. and Koekemoer, A. and Dayal, P. and Curtis-Lake, E. and Wild, V. and Charlot, S. and Bowler, R. A. A. and Schenker, M. A. and Ouchi, M. and Ono, Y. and Cirasuolo, M. and Furlanetto, S. R. and Stark, D. P. and Targett, T. A. and Schneider, E. (2013) The UV continua and inferred stellar populations of galaxies at z ≃ 7–9 revealed by the Hubble Ultra-Deep Field 2012 campaign. Monthly Notices of the Royal Astronomical Society, 432 (4). pp. 3520-3533. ISSN 0035-8711.

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We use the new ultra-deep, near-infrared imaging of the Hubble Ultra-Deep Field (HUDF) provided by our UDF12 Hubble Space Telescope (HST) Wide Field Camera 3/IR campaign to explore the rest-frame ultraviolet (UV) properties of galaxies at redshifts z > 6.5. We present the first unbiased measurement of the average UV power-law index, 〈β〉, (fλ ∝ λ^β) for faint galaxies at z ≃ 7, the first meaningful measurements of 〈β〉 at z ≃ 8, and tentative estimates for a new sample of galaxies at z ≃ 9. Utilizing galaxy selection in the new F140W (J_140) imaging to minimize colour bias, and applying both colour and power-law estimators of β, we find 〈β〉 = −2.1 ± 0.2 at z ≃ 7 for galaxies with M_UV ≃ −18. This means that the faintest galaxies uncovered at this epoch have, on average, UV colours no more extreme than those displayed by the bluest star-forming galaxies at low redshift. At z ≃ 8 we find a similar value, 〈β〉 = −1.9 ± 0.3. At z ≃ 9, we find 〈β〉 = −1.8 ± 0.6, essentially unchanged from z ≃ 6 to 7 (albeit highly uncertain). Finally, we show that there is as yet no evidence for a significant intrinsic scatter in β within our new, robust z ≃ 7 galaxy sample. Our results are most easily explained by a population of steadily star-forming galaxies with either ≃ solar metallicity and zero dust, or moderately sub-solar (≃10–20 per cent) metallicity with modest dust obscuration (AV ≃ 0.1–0.2). This latter interpretation is consistent with the predictions of a state-of-the-art galaxy-formation simulation, which also suggests that a significant population of very-low metallicity, dust-free galaxies with β ≃ −2.5 may not emerge until M_UV > −16, a regime likely to remain inaccessible until the James Webb Space Telescope.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Ellis, R. S.0000-0001-7782-7071
Robertson, B. E.0000-0002-4271-0364
Koekemoer, A.0000-0002-6610-2048
Ouchi, M.0000-0002-1049-6658
Furlanetto, S. R.0000-0002-0658-1243
Additional Information:© 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2013 April 22. Received 2013 April 22; in original form 2012 December 5. First published online: May 27, 2013. JSD and PD thank their collaborators U. Maio and B. Ciardi for making available the SPH simulations utilized in Section 5.3. JSD, PD, VW, RAAB and TAT acknowledge the support of the European Research Council via the award of an Advanced Grant. JSD and RJM acknowledge the support of the Royal Society via a Wolfson Research Merit Award, and a University Research Fellowship respectively. ABR and EFCL acknowledge the support of the UK Science & Technology Facilities Council. US authors acknowledge financial support from the Space Telescope Science Institute under award HST-GO-12498.01-A. SRF is partially supported by the David and Lucile Packard Foundation. SC acknowledges the support of the European Commission through the Marie Curie Initial Training Network ELIXIR. This work is based in part on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc, under NASA contract NAS5-26555. This work is also based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407.
Funding AgencyGrant Number
European Research Council (ERC)UNSPECIFIED
Science and Technology Facilities Council (STFC)UNSPECIFIED
David and Lucile Packard FoundationUNSPECIFIED
Marie Curie FellowshipUNSPECIFIED
Subject Keywords:galaxies: evolution; galaxies: formation; galaxies: high-redshift; galaxies: stellar content; dark ages, reionization, first stars
Issue or Number:4
Record Number:CaltechAUTHORS:20130807-115947121
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:39804
Deposited By: Jason Perez
Deposited On:07 Aug 2013 23:06
Last Modified:09 Mar 2020 13:19

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