Determining the Redshift of Reionization From the Spectra of High-Redshift Sources

The redshift at which the universe was reionized is currently unknown. We examine the optimal strategy for extracting this redshift, z_{reion}, from the spectra of early sources. For a source located at a redshift z_s beyond but close to reionization, 1<[(1+z_s)/(1+z_{reion})]<(32/27), the Gunn-Peterson trough splits into disjoint Lyman-alpha, beta, and possibly higher Lyman series troughs, with some transmitted flux in between these troughs. We show that although the transmitted flux is suppressed considerably by the dense Lyman-alpha forest after reionization, it is still detectable for sufficiently bright sources and can be used to infer the reionization redshift. The Next Generation Space Telescope will reach the spectroscopic sensitivity required for the detection of such sources.