On the photoionization of the intergalactic medium by quasars at high redshift

We discuss the reionization of the intergalactic medium (IGM) by quasars at high redshift. We compute the integrated UV background from observed QSOs, taking into account the hydrogen opacity associated with intervening Lyα clouds and Lyman limit systems. We note that the published data appear to indicate a significant underdensity of absorption systems in the Lyα forest with column densities NHI_ > 10¹5^ cm^-2^. This deficit, if real, would result in a reduction of the opacity of the universe at high redshift by a factor of 1. 5-3 relative to previous estimates. The QSO contribution over the range z = 3-5 to the metagalactic flux at 912 A may then be as large as 6 (1+z) /4. 5⁰. 5^ X 10^-22^ ergs cm^-2^ s^-1^ Hz^-1^ sr^-1^ for q₀_= 0, and 3 x 10^-22^ ergs cm^-2^ s^-1^ Hz^-1^ sr^-1^ for q₀_ = 0. 5. We show that this ionizing flux is consistent with current 1 σ upper limits to the Gunn-Peterson optical depth τGP_ at z ~ 3 if the density of the diffuse component of the IGM satisfies OMEGADₕ₅0_²^ ~ 3. 8 only in the case of a highly clumped medium. We argue that the Lyα forest may contain a large fraction of the baryons in the universe and estimate the corresponding mass density parameter to be in the range 0. 002 < OMEGALyαₕ₅0_ < 0. 05. We also set constraints on an alternative scenario in which the "intercloud" medium is collisionally ionized, and QSOs photoionize the Lyα forest alone. We conclude that, within the uncertainties, the observed QSOs can provide the required number of ionizing photons at early epochs in several of the models examined. If quasars turn on suddenly, the universe will be completely photoionized within DELTAz <~ 0. 5 of the turn-on redshift. If instead, QSOs turn on gradually, the associated H II regions may not have fully percolated until z ~ 5-5. 5. We argue that a new class of absorption systems, arising from intervening patches of neutral unprocessed material which have not yet been engulfed by an H II region, could be observed in the spectra of QSOs just beyond this breakthrough epoch. We estimate their typical H I column densities, sizes, and velocity widths to be 10¹8^-10²0^ cm^-2^, 0. 1-1 Mpc, and 100-1000 km s^-1^. The patches are expected to persist as distinct absorption features over an extended redshift interval of width DELTAz ~ 0. 5-1. Eventually, the increase of the equivalent widths of the lines with redshift results in their collective formation of a Gunn-Peterson absorption trough shortward of the Lyα emission line. We find that, for most successful models, the patches appear when τGP_ is still less than unity. Because of the difficulty of measuring such small optical depths at high z, due to the presence of the Lyα forest, neutral patches along the line of sight may provide the first unambiguous detection of a diffuse, primordial IGM. The appearance of these systems would be a sensitive probe of the epoch of reionization. We also discuss the radio signatures of the lines. Although our emphasis is on the photoionization of the IGM by QSOs, the formalism we develop, including the effects of UV attenuation by intervening absorption systems, as well as many of our results, applies to any discrete source of photoionizing radiation of known number density, intensity, and spectrum.