Abstract / Description of output
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, , (f(lambda) alpha lambda(beta)) for faint galaxies at z similar or equal to 7, the first meaningful measurements of at z similar or equal to 8, and tentative estimates for a new sample of galaxies at z similar or equal to 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 = -2.1 +/- 0.2 at z similar or equal to 7 for galaxies with M-UV similar or equal to -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 similar or equal to 8 we find a similar value, = -1.9 +/- 0.3. At z similar or equal to 9, we find = -1.8 +/- 0.6, essentially unchanged from z similar or equal to 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 similar or equal to 7 galaxy sample. Our results are most easily explained by a population of steadily star-forming galaxies with either similar or equal to solar metallicity and zero dust, or moderately sub-solar (similar or equal to 10-20 per cent) metallicity with modest dust obscuration (A(V) similar or equal to 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 similar or equal to -2.5 may not emerge until M-UV > -16, a regime likely to remain inaccessible until the James Webb Space Telescope.
Original language | English |
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Pages (from-to) | 3520-3533 |
Number of pages | 14 |
Journal | Monthly Notices of the Royal Astronomical Society |
Volume | 432 |
Issue number | 4 |
DOIs | |
Publication status | Published - 1 Jul 2013 |
Keywords / Materials (for Non-textual outputs)
- galaxies: evolution
- galaxies: formation
- galaxies: high-redshift
- galaxies: stellar content
- dark ages, reionization, first stars