The UV continua and inferred stellar populations of galaxies at z ≃ 7-9 revealed by the Hubble Ultra-Deep Field 2012 campaign
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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 α λβ) for faint galaxies at z ≃ 7, the first meaningful measurements of at z ≃ 8, and tentative estimates for a new sample of galaxies at z ≃ 9. Utilizing galaxy selection in the new F140W (J140) 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 MUV ≃ -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 MUV > -16, a regime likely to remain inaccessible until the James Webb Space Telescope.
Dunlop , J S , Rogers , A B , McLure , R J , Ellis , R S , Robertson , B E , Koekemoer , A , Dayal , P , Curtis-Lake , E , Wild , V , Charlot , S , Bowler , R A A , Schenker , M A , Ouchi , M , Ono , Y , Cirasuolo , M , Furlanetto , S R , Stark , D P , Targett , T A & 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 , vol 432 , no. 4 , pp. 3520-3533 . DOI: 10.1093/mnras/stt702
Monthly Notices of the Royal Astronomical Society
Copyright, 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.
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.
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