Evolution in the escape fraction of ionizing photons and the decline in strong Lyα emission from z > 6 galaxies

The rapid decline in the number of strong Ly alpha emitting galaxiesat z > 6 provides evidence for neutral hydrogen in the intergalactic medium, but is difficult to explain with plausible models for reionization. We demonstrate that the observed reduction in Ly alpha flux from galaxies at z > 6 can be explained by evolution in the escape fraction of ionizing photons, f(esc). We find that the median observed drop in the fraction of galaxies showing strong Ly alpha emission, as well as the observed evolution of the Ly alpha luminosity function both follow from a small increase in f(esc) of Delta f(esc) 0.1 from f(esc) similar to 0.6 at z similar to 6. This high escape fraction may be at odds with current constraints on the ionizing photon escape fraction, which favour smaller values of f(esc) less than or similar to 20 per cent. However, models that invoke a redshift evolution of f(esc) that is consistent with these constraints can suppress the z similar to 7 Ly alpha flux to the observed level, if they also include a small evolution in global neutral fraction of Delta x(HI) similar to 0.2. Thus, an evolving escape fraction of ionizing photons can be a plausible part of the explanation for evolution in the Ly alpha emission of high-redshift galaxies. More generally, our analysis also shows that the drop in the Ly alpha fraction is quantitatively consistent with the observed evolution in the Ly alpha luminosity functions of Ly alpha emitters.