Publication Date:
2013-06-18
Description:
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〉, ( f 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 ( A V ~= 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 .
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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