A flat photoionization rate at 2≤z≤4.2:: Evidence for a stellar-dominated UV background and against a decline of cosmic star formation beyond z∼3

We investigate the implications of our measurement of the Ly alpha forest opacity at redshifts from a 2 <= z <= 4.2 sample of 86 high-resolution quasar spectra for the evolution of the cosmic ultraviolet luminosity density and its sources. The derived hydrogen photoionization rate Gamma is remarkably flat over this redshift range, implying an increasing comoving ionizing emissivity with redshift. Because the quasar luminosity function is strongly peaked near, z similar to 2 star-forming galaxies likely dominate the ionizing emissivity at z greater than or similar to 3. Our measurement argues against a star formation rate density declining beyond, in contrast to existing state-of-the-art determinations of the z similar to 3 cosmic star formation history from direct galaxy counts. Stellar emission from galaxies therefore likely reionized the universe.