2011MNRAS.414.2139D

We study the visibility of the Lyα emission line during the epoch of reionization. Combining galactic outflow models with large-scale seminumeric simulations of reionization, we quantify the probability distribution function of the fraction of Lyα photons transmitted through the intergalactic medium (IGM), T_IGM. Our study focuses on galaxies populating dark matter haloes with masses of M_halo= 10¹⁰ M_☉ at z= 8.6, which is inspired by the recent reported discovery of a galaxy at z= 8.6 with strong Lyα line emission. For reasonable assumptions, we find that the combination of winds and reionization morphology results in T_IGM > rsim10 per cent (50 per cent), for the majority of galaxies, even when the Universe is ∼80 per cent (60 per cent) neutral by volume. Thus, the observed strong Lyα emission from the reported z= 8.6 galaxy is consistent with a highly neutral IGM, and cannot be used to place statistically significant constraints on the volume-averaged neutral fraction of hydrogen in the IGM. We also investigate the implications of the recent tentative evidence for an observed decrease in the `Lyman alpha emitter fraction' among drop-out galaxies between z= 6 and 7. If confirmed, we show that a rapid evolution in x_Hii will be required to explain this observation via the effects of reionization.