Evolution of the Intergalactic Opacity: Implications for the Ionizing Background, Cosmic Star Formation, and Quasar Activity

Abstract

We investigate the implications of the intergalactic opacity for the evolution of the cosmic UV luminosity density and its sources. Our main constraint is our measurement of the Ly? forest opacity at redshifts -->2 ? z ? 4.2 from 86 high-resolution quasar spectra. In addition, we impose the requirements that H I must be reionized by -->z = 6 and He II by -->z ~ 3 and consider estimates of the hardness of the ionizing background from H I-to-He II column density ratios. The derived hydrogen photoionization rate is remarkably flat over the Ly? forest redshift range covered. Because the quasar luminosity function is strongly peaked near -->z ~ 2, the lack of redshift evolution indicates that star-forming galaxies likely dominate the photoionization rate at -->z 3. Combined with direct measurements of the galaxy UV luminosity function, this requires only a small fraction -->fesc ~ 0.5% of galactic hydrogen-ionizing photons to escape their source for galaxies to solely account for the entire ionizing background. Under the assumption that the galactic UV emissivity traces the star formation rate, current state-of-the-art observational estimates of the star formation rate density appear to underestimate the total photoionization rate at -->z ~ 4 by a factor of ~4, are in tension with recent determinations of the UV luminosity function, and fail to reionize the universe by -->z ~ 6 if extrapolated to arbitrarily high redshift. A theoretical star formation history peaking earlier fits the Ly? forest photoionization rate well, reionizes the universe in time, and is in better agreement with the rate of -->z ~ 4 gamma-ray bursts observed by Swift. Quasars suffice to doubly ionize helium by -->z ~ 3 and likely contribute a nonnegligible and perhaps dominant fraction of the hydrogen-ionizing background at their -->z ~ 2 peak.