Constraints on the ionization sources of the high redshift intergalactic   medium
  Constraints on the ionization structure of the Intergalactic Medium 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 UV ionizing background at z<6 is the same population which reionized the IGM, it is shown that consistency with measurements of the mean LyA transmitted flux at high redshifts requires the epoch of hydrogen reionization of the IGM to have occurred at a redshift z < 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 WMAP requires the sources of photoionization to have had hard spectra, such as QSOs or Population III stars. The only way to escape these conclusions is either: 1.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 2.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 contribution of QSOs to the UV background is also estimated based on recent surveys. 
A. Meiksin