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 Lya forest opacity at redshifts 2 ≤ z ≤ 4.2 from a sample of 86 high-resolution quasar spectra for the evolution of the cosmic ultraviolet luminosity density and its sources. The derived hydrogen photoionization rate G 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 ∼ 2, star-forming galaxies likely dominate the ionizing emissivity atz ≳ 3. Our measurement argues against a star formation rate density declining beyondz ∼ 3, in contrast to existing state-of-the-art determinations of the cosmic star formation history from direct galaxy counts. Stellar emission from galaxies therefore likely reionized the universe.