The UV continua and inferred stellar populations of galaxies at z ~ 7 -
  9 revealed by the Hubble Ultra Deep Field 2012 campaign
  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.
