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Date and Time of the Query: 2019-06-25 T22:00:57 PDT
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For refcode 2013MNRAS.432.3520D:
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Copyright by Royal Astronomical Society. 2013MNRAS.432.3520D 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.; 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. Abstract. 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, <beta>, (f_lambda_ is proportional to lambda^beta^) for faint galaxies at z ~= 7, the first meaningful measurements of <beta> 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 beta, we find <beta> = -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, <beta> = -1.9 +/- 0.3. At z ~= 9, we find <beta> = -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 beta 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 beta ~= -2.5 may not emerge until M_UV_ > -16, a regime likely to remain inaccessible until the James Webb Space Telescope. Key words: galaxies: evolution, galaxies: formation, galaxies: high-redshift, galaxies: stellar content, dark ages, reionization, first stars
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