Constraints on the ionization sources of the high-redshift intergalactic medium

Constraints on the ionization structure of the intergalactic medium (IGM) are derived as directly imposed by observations in conjunction with the results of numerical simulations for structure formation. Under the assumption that the population of sources dominating the ultraviolet (UV) ionizing background at z < 6 is the same population which reionized the IGM, it is shown that consistency with measurements of the mean Ly alpha transmitted flux at high redshifts requires the epoch of hydrogen reionization of the IGM to have occurred at a redshift z(ri) < 11, independent of the space density of the sources. The upper limit on the reionization redshift depends only on the shape of the UV spectra of the sources. Consistency with constraints on the reionization epoch from the Wilkinson Microwave Anisotropy Probe requires the sources of photoionization to have had hard spectra, such as quasi-stellar objects (QSOs) or Population III stars. The only ways to escape these conclusions are either (i) the sources which dominate the photoionization background at z < 6 are the remnants of a population that was a much more prodigious source of ionizing photons at earlier times, or (ii) the sources responsible for the photoionization of the IGM are an unknown population that contributes negligibly to the ionization of the IGM at z < 6. The evolution of the QSO luminosity function over the range 3 < z < 6 is estimated from recent QSO counts, and the fraction of ionizing photons arising in QSOs is evaluated under the assumptions of either pure luminosity evolution or pure density evolution. It is shown that QSOs dominate the UV ionizing background for z < 3.5, but that it is unlikely that more than one-half to one-third of the ionizing background radiation originates in QSOs in the redshift range 4.5 < z < 6. QSOs acting alone could not have reionized the hydrogen in the IGM prior to z approximate to 4. If the QSOs had hard spectra, however, they may have reionized the helium in the IGM as early as z approximate to 5. The possibility that the IGM was reionized by low-luminosity active galactic nuclei is discussed.