The effects of reionization on Lyα galaxy surveys

Searches for Ly alpha emission lines are among the most effective ways to identify high-redshift galaxies. They are particularly interesting because they probe not only the galaxies themselves but also the ionization state of the intergalactic medium: nearby neutral gas efficiently absorbs Ly alpha photons. The observed linestrengths depend on the amount by which each photon is able to redshift away from line centre before encountering neutral gas and hence on the size distribution of HII regions surrounding the sources. Here, we use an analytic model of that size distribution to study the effects of reionization on the luminosity function of Ly alpha emitters and their observed spatial distribution. Our model includes the clustering of high-redshift galaxies and thus contains ionized bubbles much larger than those expected around isolated galaxies. As a result, Ly alpha-emitting galaxies remain visible earlier in reionization: we expect the number counts to decline by only a factor of similar to 2 ( or 10) when the mean ionized fraction falls to (x) over bar (i) similar to 0.75 (or 0.5) in the simplest model. Moreover, the absorption is not uniform across the sky: galaxies remain visible only if they sit inside large bubbles, which become increasingly rare as (x) over bar (i) decreases. Thus, the size distribution also affects the apparent clustering of Ly alpha-selected galaxies. On large scales, it traces that of the large bubbles, which in our model are more biased than the galaxies. On small scales, the clustering increases rapidly as (x) over bar (i) decreases because large HII regions surround strong galaxy overdensities, so a survey automatically selects only those galaxies with neighbours. The transition between these two regimes occurs at the characteristic bubble size. Hence, large Ly alpha galaxy surveys have the potential to measure directly the size distribution of HII regions during reionization.