Abstract
We combine high resolution hydrodynamical simulations with an intermediate
resolution, dark matter only simulation and an analytical model for the growth
of ionized regions to estimate the large scale distribution and redshift
evolution of the visibility of Ly$\alpha$ emission in $6 z8$
galaxies. The inhomogeneous distribution of neutral hydrogen during the
reionization process results in significant fluctuations in the Ly$\alpha$
transmissivity on large scales, and is sensitive not only to the ionized
fraction of the intergalactic medium by volume and amplitude of the local
ionizing background, but also to the relative velocity shift of the Ly$\alpha$
emission line due to resonant scattering. We reproduce a decline in the space
density of Ly$\alpha$ emitting galaxies as rapid as observed with a
volume-weighted neutral fraction of $30$ (50) per cent at $z=7$ ($z=8$),
and a typical Ly$\alpha$ line velocity offset of $100\rm\,km\,s^-1$ redward
of systemic at $z=6$ which decreases toward higher redshift. The latest
preliminary (12/2014) Planck results indicate such a recent end to reionization
is no longer disfavoured by constraints from the cosmic microwave background.
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