EVOLUTION OF THE INTERGALACTIC OPACITY: IMPLICATIONS FOR THE IONIZING BACKGROUND, COSMIC STAR FORMATION, AND QUASAR ACTIVITY

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 alpha 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 similar to 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 alpha forest redshift range covered. Because the quasar luminosity function is strongly peaked near z similar to 2, the lack of redshift evolution indicates that star-forming galaxies likely dominate the photoionization rate at z greater than or similar to 3. Combined with direct measurements of the galaxy UV luminosity function, this requires only a small fraction f(esc) similar to 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 similar to 4 by a factor of similar to 4, are in tension with recent determinations of the UV luminosity function, and fail to reionize the universe by z similar to 6 if extrapolated to arbitrarily high redshift. A theoretical star formation history peaking earlier fits the Ly alpha forest photoionization rate well, reionizes the universe in time, and is in better agreement with the rate of z similar to 4 gamma-ray bursts observed by Swift. Quasars suffice to doubly ionize helium by z similar to 3 and likely contribute a nonnegligible and perhaps dominant fraction of the hydrogenionizing background at their z similar to 2 peak.