Reionization of the Intergalactic Medium and the Damping Wing of the Gunn-Peterson Trough

Abstract

Observations of high-redshift quasars show that the IGM must have been reionized at some redshift $z>5$. If a source of radiation could be observed at the rest-frame Lya wavelength, at a sufficiently high redshift where some of the IGM in the line-of-sight was not yet reionized, the Gunn-Peterson trough should be present. Longward of the Lya wavelength, a damping wing should be observed caused by the neutral IGM whose absorption profile can be predicted. Measuring the shape of this damping wing would provide irrefutable evidence of the observation of the IGM before reionization, and a determination of the density of the neutral IGM. This measurement might be hindered by the possible presence of a dense absorption system associated with the source. Shortward of the \lya wavelength, absorption should be seen from the patchy structure of the IGM in the process of reionization. We show that a complete Gunn-Peterson trough is most likely to continue to be observed through the epoch where the IGM is partially ionized. The damping wings of the neutral patches should overlap if the proper pathlength through an ionized region is less than 1 h^{-1} Mpc; even in larger ionized regions, the characteristic background intensity should be low enough to yield a very high optical depth due to the residual neutral fraction, although occasionally some flux may be transmitted through large, underdense voids within an ionized region. The case of the HeII reionization is also discussed, and we argue that helium was already doubly ionized by z=3 throughout the IGM. The recently discovered afterglows of gamma-ray bursts might soon be observed at very high redshifts. Their featureless continuum spectrum and high luminosities make them ideal sources for studying absorption by the IGM.