On the rapid demise of Ly α emitters at redshift z ≳ 7 due to the increasing incidence of optically thick absorption systems

A variety of independent observational studies have now reported a significant decline in the fraction of Lyman-break galaxies which exhibit Ly alpha emission over the redshift interval z = 6-7. In combination with the strong damping wing extending redward of Ly alpha 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 similar to 7. Current theoretical models imply H I fractions as large as 40-90 per cent may be required to explain these data assuming there is no intrinsic evolution in the Ly alpha 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 similar to 6-7) quasars, we demonstrate that the opacity of the intervening IGM redward of rest-frame Ly alpha 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 per cent 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.