Directly Observing the Galaxies Likely Responsible for Reionization

We report a new analysis of the Hubble Frontier Fields clusters Abell 2744 and MACS 0416 using wavelet decomposition to remove the cluster light, enabling the detection of highly magnified (>50×) galaxies a factor of 10× fainter in luminosity than previous studies. We find 167 galaxies at z≳ 6, and with this sample we are able to characterize the UV luminosity function to {M}_{UV}=-12.5 at z∼ 6, -14 at z∼ 7, and -15 at z∼ 8. We find a steep faint-end slope (α < -2), and with our improved statistics at the faint end we reduce the fractional uncertainty on α to < 2 % at z∼ 6{--}7 and 4% at z∼ 8. We also investigate the systematic uncertainty due to the lens modeling by using every available lens model individually and comparing the results; this systematic fractional uncertainty on α is < 4 % at all redshifts. We now directly observe galaxies in the luminosity regime where some simulations predict a change in the faint-end slope of the luminosity function, yet our results provide statistically very strong evidence against any turnover in the luminosity range probed, more consistent with simulations in which stars form in lower-mass halos. Thus, we find strong support for the extension of the steep luminosity function to {M}_{UV}=-13 at z> 6, consistent with the number of faint galaxies needed to reionize the universe under standard assumptions.