  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 (Dayal et al. 2018, Mason et al. 2019, Robertson et al. 2022), has
been a matter of intense debate (Robertson et al. 2022). Some models suggest
that high ionizing emissivity and escape fractions ($f_{\rm esc}$) from quasars
support their role in driving cosmic reionization (Madau & Haardt 2015, Mitra
et al. 2018). Others propose that the high $f_{\rm esc}$ values from bright
galaxies generates sufficient ionizing radiation to drive this process (Naidu
et al. 2020). Finally, a few studies suggest that the number density of faint
galaxies, when combined with a stellar-mass-dependent model of ionizing
efficiency and $f_{\rm esc}$, can effectively dominate cosmic reionization
(Finkelstein et al. 2019, Dayal et al. 2020). However, so far, low-mass
galaxies have eluded comprehensive spectroscopic studies owing to 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
$M_{\rm UV}$ $\sim -17$ to $-15$ mag (down to 0.005 $L^{\star}$. We find that
faint galaxies during the Universe's first billion years produce ionizing
photons with log($\xi_{\mathrm{ion}}$/ Hz erg$^{-1}$) =$25.80\pm 0.14$, a
factor of 4 higher than commonly assumed values (Robertson et al. 2015). 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 order five per cent.
