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

Abstract

The rapid decline in the number of strong Lyα 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 Lyα flux from galaxies at z > 6 can be explained by evolution in the escape fraction of ionizing photons, fesc. We find that the median observed drop in the fraction of galaxies showing strong Lyα emission, as well as the observed evolution of the Lyα luminosity function both follow from a small increase in fesc of Δfesc ∼ 0.1 from fesc ∼ 0.6 at z ∼ 6. This high escape fraction may be at odds with current constraints on the ionizing photon escape fraction, which favour smaller values of fesc ≲ 20 per cent. However, models that invoke a redshift evolution of fesc that is consistent with these constraints can suppress the z ∼ 7 Lyα flux to the observed level, if they also include a small evolution in global neutral fraction of ΔxH I ∼ 0.2. Thus, an evolving escape fraction of ionizing photons can be a plausible part of the explanation for evolution in the Lyα emission of high-redshift galaxies. More generally, our analysis also shows that the drop in the Lyα fraction is quantitatively consistent with the observed evolution in the Lyα luminosity functions of Lyα emitters.