The Detectability of High Redshift Lyman Alpha Emission Lines Prior to the Reionization of the Universe

For a source of Ly alpha radiation embedded in a neutral intergalactic medium (IGM) prior to the reionization epoch, the 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 HII region surrounding the source, and therefore with the ionizing luminosity and age of the source. Motivated by the recent discovery of a Ly alpha emitting galaxy at a redshift z=6.56 (Hu et al. 2002), possibly prior to the reionization of the IGM, we revisit the effects of a neutral IGM on the Ly alpha emission line. We show that even for faint sources with little ionizing continuum, the emission line can 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 Ly alpha line as functions of the ionizing emissivity of its source. A statistical sample of Ly alpha emitters extending beyond the reionization redshift can be a useful probe of reionization.