Most of the photons that reionized the Universe came from dwarf galaxies.

The identification of sources driving cosmic reionization, a major phase transition from neutral hydrogen to ionized plasma around 600-800 Myr after the Big Bang 1-3 , has been a matter of debate 4 . Some models suggest that high ionizing emissivity and escape fractions (f esc ) from quasars support their role in driving cosmic reionization 5,6 . Others propose that the high f esc values from bright galaxies generate sufficient ionizing radiation to drive this process 7 . Finally, a few studies suggest that the number density of faint galaxies, when combined with a stellar-mass-dependent model of ionizing efficiency and f esc , can effectively dominate cosmic reionization 8,9 . However, so far, comprehensive spectroscopic studies of low-mass galaxies have not been done because of their extreme faintness. Here we report an analysis of eight ultra-faint galaxies (in a very small field) during the epoch of reionization with absolute magnitudes between M UV  ≈ -17 mag and -15 mag (down to 0.005L ⋆ (refs.  10,11 )). We find that faint galaxies during the first thousand million years of the Universe produce ionizing photons with log[ξ ion  (Hz erg -1 )] = 25.80 ± 0.14, a factor of 4 higher than commonly assumed values 12 . If this field is representative of the large-scale distribution of faint galaxies, the rate of ionizing photons exceeds that needed for reionization, even for escape fractions of the order of 5%.