Lyα emitters during the early stages of reionization

We investigate the potential of exploiting Lya Emitters (LAEs) to constrain the volume-weighted mean neutral hydrogen fraction of the IGM, xH, at high redshifts (specifically z~9). We use "semi-numerical'' simulations to efficiently generate density, velocity, and halo fields at z=9 in a 250 Mpc box, resolving halos with masses M>2. 2e8 solar masses. We construct ionization fields corresponding to various values of xH. With these, we generate LAE luminosity functions and "counts-in-cell'' statistics. As in previous studies, we find that LAEs begin to disappear rapidly when xH > 0. 5. Constraining xH (z=9) with luminosity functions is difficult due to the many uncertainties inherent in the host halo mass Lya luminosity mapping. However, using a very conservative mapping, we show that the number densities derived using the six z~9 LAEs recently discovered by Stark et al. (2007) imply xH 10% higher probability of containing more than one LAE in a xH>0. 5 universe than a xH=0 universe with the same overall number density. With this statistic, a fully ionized universe can be robustly distinguished from one with xH > 0. 5 using a survey containing only ~ 20--100 galaxies.