James S. Bolton, Martin G. Haehnelt
A variety of independent observational studies have now reported a significant decline in the fraction of Lyman-break galaxies which exhibit Ly-a emission over the redshift interval z=6-7. In combination with the strong damping wing extending redward of Ly-a 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 HI fractions as large as 40-90 per cent may be required to explain these data assuming there is no intrinsic evolution in the Ly-a 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 Ly-a can rise rapidly in average regions of the Universe simply because of the increasing incidence of optically thick absorption systems as the tail-end of reionisation is approached. Our simulations suggest that a volume-averaged neutral fraction of ~10 per cent is sufficient to explain the Ly-a emitter fraction at z=7, assuming the observed decline is due to evolution in the IGM HI fraction only. These data do not require a large change in the IGM neutral fraction by several tens of per cent from z=6-7, but may instead be indicative of the rapid decrease in the typical mean free path for ionising photons expected during the final stages of reionisation.
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http://arxiv.org/abs/1208.4417
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