The evolution of the comoving production rate of ionizing photons. The required rate inferred from measurements of the mean transmitted Lyα flux is shown by the solid line. The long-dashed line shows the required source rate if diffuse recombination radiation from the IGM boosts the total metagalactic photoionization rate by 50 per cent. Also shown are the minimal ionizing photon production rate nHt1H required to ionize the IGM over a Hubble time (dotted line), and the rate of recombinations for clumping factors of