The UV continua and inferred stellar populations of galaxies at z ~ 7 - 9 revealed by the Hubble Ultra Deep Field 2012 campaign
A paper on "The UV continua and inferred stellar populations of galaxies at z ~ 7 - 9 revealed by the Hubble Ultra Deep Field 2012 campaign" by J. S. Dunlop et al. has been submitted to the Monthly Notices of the Royal Astronomical Society. A preprint is available at http://arxiv.org/abs/1212.0860:
The UV continua and inferred stellar populations of galaxies at z ~ 7 - 9 revealed by the Hubble Ultra Deep Field 2012 campaign
Authors:
J. S. Dunlop, A. B. Rogers, R. J. McLure, R. S. Ellis, B. E. Robertson, A. Koekemoer, P. Dayal, E. Curtis-Lake, V. Wild, S. Charlot, R. A. A. Bowler, M. A. Schenker, M. Ouchi, Y. Ono, M. Cirasuolo, S. R. Furlanetto, D. P. Stark, T. A. Targett, E. Schneider
Abstract:
We use the new ultra-deep, near-infrared imaging of the Hubble Ultra-Deep Field (HUDF) provided by our UDF12 HST WFC3/IR campaign to explore the rest-frame UV properties of galaxies at redshifts z > 6.5. We present the first unbiased measurement of the average UV power-law index, 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. Utilising galaxy selection in the new F140W 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 - 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%) 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.
The Hubble Ultra Deep Field 2012 program is a collaboration between the California Institute of Technology, the University of Arizona, the University of Edinburgh, the Space Telescope Science Institute, the University of California Los Angeles, the University of Tokyo, and the Institut Astrophysique de Paris. Support for this Program was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research In Astronomy, Incorporated, under NASA contract NAS5-26555.
