2013MNRAS.429.1695B

A variety of independent observational studies have now reported a significant decline in the fraction of Lyman-break galaxies which exhibit Lyα emission over the redshift interval z = 6-7. In combination with the strong damping wing extending redward of Lyα in the spectrum of the bright z = 7.085 quasar ULAS 1120+0641, this has strengthened suggestions that the hydrogen in the intergalactic medium (IGM) is still substantially neutral at z ∼ 7. Current theoretical models imply H i fractions as large as 40-90 percent may be required to explain these data assuming there is no intrinsic evolution in the Lyα emitter population. We propose that such large neutral fractions are not necessary. Based on a hydrodynamical simulation which reproduces the absorption spectra of high-redshift (z ∼ 6-7) quasars, we demonstrate that the opacity of the intervening IGM redward of rest-frame Lyα can rise rapidly in average regions of the Universe simply because of the increasing incidence of absorption systems which are optically thick to Lyman continuum photons as the tail-end of reionization is approached. Our simulations suggest that these data do not require a large change in the IGM neutral fraction by several tens of percent from z = 6 to 7, but may instead be indicative of the rapid decrease in the typical mean free path for ionizing photons expected during the final stages of reionization.