2008ApJ...682L...9F

We investigate the implications of our measurement of the Lyα 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 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.