Lyα emitters during the early stages of reionization

We investigate the potential of exploiting Ly alpha emitters (LAEs) to constrain the volume-weighted mean neutral hydrogen fraction of the intergalactic medium, (x) over bar (H) (I), at high redshifts (z similar to 9). We use 'seminumerical' simulations to efficiently generate density, velocity and halo fields at z = 9 in a 250-Mpc box, resolving haloes with masses M >= 2.2 x 10(8) M(circle dot). We construct ionization fields corresponding to various values of (x) over bar (H) (I). 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 (x) over bar (H) (I) greater than or similar to 0.5. Constraining (x) over bar (H) (I)(z = 9) with luminosity functions is difficult due to the many uncertainties inherent in the host halo mass <-> Ly alpha luminosity mapping. However, using a very conservative mapping, we show that the number densities derived using the six z similar to 9 LAEs recently discovered by Stark et al. (2007a) imply (x) over bar (H) (I) less than or similar to 0.7. On a more fundamental level, these LAE number densities, if genuine, require substantial star formation in haloes with M less than or similar to 10(9) M(circle dot), making them unique among the current sample of observed high-z objects. Furthermore, reionization increases the apparent clustering of the observed LAEs. We show that a 'counts-in-cell' statistic is a powerful probe of this effect, especially in the early stages of reionization. Specifically, we show that a field of view (typical of upcoming infrared instruments) containing LAEs has greater than or similar to 10 per cent higher probability of containing more than one LAE in a (x) over bar (H) (I) greater than or similar to 0.5 universe than a (x) over bar (H) (I) approximate to 0 universe with the same overall number density. With this statistic, an ionized universe can be robustly distinguished from one with (x) over bar (H) (I) greater than or similar to 0.5 using a survey containing only similar to 20-100 galaxies.