Abstract
The hypothesis that photoionization of the IGM by high redshift radiation sources explains the absence of a detectable Gunn-Peterson effect in the spectra of all known quasars is investigated. This hypothesis has strong implications for the epoch and nature of galaxy formation as it relates to the origin of the required ionizing radiation background. New calculations are described of the ionization and thermal history of the IGM which determine the minimum required ionizing photon emissivity of the universe at z ≳ 4. The inability of the observed high redshift quasars to produce this emissivity is discussed, as are the consequences for primordial galaxy luminosity and metal production if early-type stars provide the emissivity instead. Other alternatives, such as radiation from protogalactic shocks and the radiative decay of unstable’inos, are also discussed.
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