Abstract
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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