The Detectability of High-Redshift L[CLC]y[/CLC]α 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 suppressedby the intervening IGM. The damping wing of the so-called Gunn-Peterson troughcan extend to the red side of the emission line, and erase a significantfraction of the total line flux. However, the transmitted fraction increaseswith the size of the local cosmological HII region surrounding the source, andtherefore with the ionizing luminosity and age of the source. Motivated by therecent 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 ofa neutral IGM on the Ly alpha emission line. We show that even for faintsources with little ionizing continuum, the emission line can remainobservable. In particular, the line detected by Hu et al. is consistent with asource embedded in a neutral IGM. We provide characterizations of the asymmetryand total transmitted flux of the Ly alpha line as functions of the ionizingemissivity of its source. A statistical sample of Ly alpha emitters extendingbeyond the reionization redshift can be a useful probe of reionization.Comment: Submitted to ApJL, 4 figures include