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We use the new ultra-deep, near-infrared imaging of the Hubble Ultra-DeepField (HUDF) provided by our UDF12 HST WFC3/IR campaign to explore therest-frame UV properties of galaxies at redshifts z > 6.5. We present the firstunbiased measurement of the average UV power-law index, beta, for faintgalaxies at z ~ 7, the first meaningful measurements of beta at z ~ 8, andtentative estimates for a new sample of galaxies at z ~ 9. Utilising galaxyselection in the new F140W imaging to minimize colour bias, and applying bothcolour and power-law estimators of beta, we find beta = -2.1 (+/-0.2) at z ~ 7for galaxies with M_UV ~ -18. This means that the faintest galaxies uncoveredat this epoch have, on average, UV colours no more extreme than those displayedby the bluest star-forming galaxies at low redshift. At z ~ 8 we find a similarvalue, 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, weshow that there is as yet no evidence for a significant intrinsic scatter inbeta within our new, robust z ~ 7 galaxy sample. Our results are most easilyexplained by a population of steadily star-forming galaxies with either ~ solarmetallicity and zero dust, or moderately sub-solar (~ 10-20%) metallicity withmodest dust obscuration (A_V ~ 0.1-0.2). This latter interpretation isconsistent with the predictions of a state-of-the-art galaxy-formationsimulation, which also suggests that a significant population of very-lowmetallicity, dust-free galaxies with beta ~ -2.5 may not emerge until M_UV >-16, a regime likely to remain inaccessible until the James Webb SpaceTelescope.

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