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We report a new analysis of the Hubble Frontier Fields clusters Abell 2744and MACS 0416 using wavelet decomposition to remove the cluster light, enablingthe detection of highly magnified (>50x) galaxies a factor of 10x fainter inluminosity than previous studies. We find 167 galaxies at z > 6, and with thissample we are able to characterize the UV luminosity function to M_UV = -12.5at z ~ 6, -14 at z ~ 7 and -15 at z ~ 8. We find a steep faint-end slope (alpha<-2), and with our improved statistics at the faint end we reduce thefractional uncertainty on alpha to <2% at z ~ 6 - 7 and 4% at z ~ 8. We alsoinvestigate the systematic uncertainty due to the lens modelling by using everyavailable lens model individually and comparing the results; this systematicfractional uncertainty on alpha is <4% at all redshifts. We now directlyobserve galaxies in the luminosity regime where simulations predict a change inthe faint-end slope of the luminosity function, yet our results providestatistically very strong evidence against any turnover in the luminosity rangeprobed, more consistent with simulations in which stars form in lower-masshalos. Thus we find strong support for the extension of the steep luminosityfunction to M_UV = -13 at z > 6, consistent with the number of faint galaxiesneeded to reionize the Universe under standard assumptions.

Directly Observing the Galaxies Likely Responsible for Reionization