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A variety of independent observational studies have now reported asignificant decline in the fraction of Lyman-break galaxies which exhibit Ly-aemission over the redshift interval z=6-7. In combination with the strongdamping wing extending redward of Ly-a in the spectrum of the bright z=7.085quasar ULAS 1120+0641, this has strengthened suggestions that the hydrogen inthe intergalactic medium (IGM) is still substantially neutral at z~7. Currenttheoretical models imply HI fractions as large as 40-90 per cent may berequired to explain these data assuming there is no intrinsic evolution in theLy-a emitter population. We propose that such large neutral fractions are notnecessary. Based on a hydrodynamical simulation which reproduces the absorptionspectra of high-redshift (z~6-7) quasars, we demonstrate that the opacity ofthe intervening IGM redward of rest-frame Ly-a can rise rapidly in averageregions of the Universe simply because of the increasing incidence ofabsorption systems which are optically thick to Lyman continuum photons as thetail-end of reionisation is approached. Our simulations suggest these data donot require a large change in the IGM neutral fraction by several tens of percent from z=6-7, but may instead be indicative of the rapid decrease in thetypical mean free path for ionising photons expected during the final stages ofreionisation.

On the rapid demise of Lyman-alpha emitters at z>7 due to the increasing incidence of optically thick absorption systems