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, zreion, from the spectra of early sources. For a source located at a redshift zs beyond but close to the reionization redshift, (1 + zreion) < (1 + zs) < 32/27(1 + zreion), the Gunn-Peterson trough splits into disjoint Lyα, Lyβ, and possibly higher Lyman series troughs, with some transmitted flux between these troughs. We show that although the transmitted flux is suppressed considerably by the dense Lyα forest at high redshifts, it could still be detectable as long as zreion ≲ 8. If the ionizing sources are bright and have a short duty cycle (e.g., if they are miniquasars or starbursts with a lifetime ≲106 yr), then breakthrough occurs shortly after the H II regions around them overlap, and the transmitted flux provides a measurement of zreion. The Next Generation Space Telescope will reach the spectroscopic sensitivity required for such a measurement.