The detectability of high-redshift Lyα emission lines prior to the reionization of the universe

For a source of Lyα radiation embedded in a neutral intergalactic medium (IGM) prior to the reionization epoch, the Lyα emission line is strongly suppressed by the intervening IGM. The damping wing of the so-called Gunn-Peterson trough can extend to the red side of the emission line and erase a significant fraction of the total line flux. However, the transmitted fraction increases with the size of the local cosmological H II region surrounding the source, and therefore with the ionizing luminosity and age of the source. Motivated by the recent discovery of a Lyα-emitting galaxy at a redshift z = 6.56 (Hu et al.), possibly prior to the reionization of the IGM, we revisit the effects of a neutral IGM on the Lyα emission line. We show that for faint sources with little or even no ionizing continuum, a sufficiently broad (Δv ≳ 300 km s^-1) emission line can still remain observable. In particular, the line detected by Hu et al. is consistent with a source embedded in a neutral IGM. We provide characterizations of the asymmetry and total transmitted flux of the Lyα line as functions of the ionizing emissivity of its source. We argue that a statistical sample of Lyα emitters extending beyond the reionization redshift can be a useful probe of reionization.