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

TL;DR

This paper argues that the observed decline in Lyman-alpha emitters at z>7 can be explained by increasing absorption systems in the IGM, without requiring a large neutral hydrogen fraction change.

Contribution

The study demonstrates through hydrodynamical simulations that the rapid decline in Lyman-alpha emission is due to absorption system incidence, not a significant increase in IGM neutrality.

Findings

Opacity of IGM rises rapidly due to absorption systems near reionisation's end.

No large change in IGM neutral fraction needed to explain observations.

Rapid decrease in mean free path of ionising photons at z>6.

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 absorption systems which are optically thick to Lyman continuum photons as the tail-end of reionisation is approached. Our simulations suggest 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.