Reionization of the intergalactic medium and the damping wing of the Gunn-Peterson trough

Observations of high-redshift quasars show that the intergalactic medium (IGM) must have been reionized at some redshift z > 5. If a source of radiation could be observed at the rest-frame Ly alpha 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 Ly alpha 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 Ly alpha 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 Ly alpha wavelength, absorption should be seen from the patchy structure of the IGM in the process of reionization, intersected in the line of sight. 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 around an ionized region should overlap in the spectrum if the proper path length through the 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. In the case of the He II reionization, the ionization fronts are much thicker than in the case of hydrogen, and the profile of this front determines the shape of the absorption at the edge of a He III region. Analogous to the case of hydrogen, windows of transmitted flux are not likely to be observed until after the low-density IGM has been completely reionized. Therefore, the observation of these transmission windows by Reimers et al. at z similar or equal to 2.85 suggests that the helium reionization was complete by this redshift. The recently discovered afterglows of gamma-ray bursts might soon be observed at the very high redshifts required for these observations. Their featureless continuum spectrum and high luminosities make them ideal sources for studying absorption by the IGM.