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 Lyman-α 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 Γ 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 at z & 3. Our measurement argues against a star formation rate density declining beyond z ∼ 3, in contrast with 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. Subject headings: Cosmology: diffuse radiation — methods: data analysis — galaxies: formation, evolution, high-redshift — quasars: absorption lines