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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 × 10 8  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 ≲ 10 9  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.

Lyα emitters during the early stages of reionization