Lyα emitters during the early stages of reionization

We investigate the potential of exploiting Lyα emitters (LAEs) to constrain the volume-weighted mean neutral hydrogen fraction of the intergalactic medium, at high redshifts (specifically z ∼ 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 × 108 M⊙. We construct ionization fields corresponding to various values of. 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. Constraining with luminosity functions is difficult due to the many uncertainties inherent in the host halo mass ↔ Lyα 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. (2007a) imply. On a more fundamental level, these LAE number densities, if genuine, require substantial star formation in haloes with M ≲ 109 M⊙, 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 ≳10 per cent higher probability of containing more than one LAE in a universe than a universe with the same overall number density. With this statistic, an ionized universe can be robustly distinguished from one with using a survey containing only ∼20-100 galaxies. © 2008 RAS