The brighter galaxies reionised the Universe

Hydrogen in the Universe was (re)ionised between redshifts $z \approx 10$ and $z \approx 6$. The nature of the sources of the ionising radiation is hotly debated, with faint galaxies below current detection limits regarded as prime candidates. Here we consider a scenario in which ionising photons escape through channels punctured in the interstellar medium by outflows powered by starbursts. We take account of the observation that strong outflows occur only when the star formation density is sufficiently high, and estimate the galaxy-averaged escape fraction as a function of redshift and luminosity from the resolved star formation surface densities in the EAGLE cosmological hydrodynamical simulation. We find that the fraction of ionising photons that escape from galaxies increases rapidly with redshift, reaching values of 5-20 percent at $z > 6$, with the brighter galaxies having higher escape fractions. Combining the dependence of escape fraction on luminosity and redshift with the observed luminosity function, we demonstrate that galaxies emit enough ionising photons to match the existing constraints on reionisation while also matching the observed UV-background post-reionisation. Our findings suggest that galaxies above the current Hubble Space Telescope detection limit emit half of the ionising radiation required to reionise the Universe.