The galaxies that reionized the Universe
  The Durham GALFORM semi-analytical galaxy formation model has been shown to
reproduce the observed rest-frame 1500\AA\ luminosity function of galaxies well
over the whole redshift range z=5-10. We show that in this model, this galaxy
population also emits enough ionizing photons to reionize the Universe by
redshift z=10, assuming a modest escape fraction of 20 per cent. The bulk of
the ionizing photons is produced in faint galaxies during starbursts triggered
by galaxy mergers. The bursts introduce a dispersion up to ~ 5 dex in galaxy
ionizing luminosity at a given halo mass. Almost 90 per cent of the ionizing
photons emitted at z=10 are from galaxies below the current observational
detection limit at that redshift. Photo-ionization suppression of star
formation in these galaxies is unlikely to affect this conclusion
significantly, because the gas that fuels the starbursts has already cooled out
of their host halos. The galaxies that dominate the ionizing emissivity at z=10
are faint, with M_{1500, AB} ~ -16, have low star formation rates, \dot{M_{*}}
~ 0.06 h^{-1} M_sun yr^{-1}, and reside in halos of mass M ~ 10^9 h^{-1} M_sun.
