The Intergalactic Lyα Opacity at 2 ≤ z ≤ 4.2: Implications for the Ionizing Background, Cosmic Star Formation, and Quasar Activity

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 ≈ (0.5 ± 0.1) × 10−12 s−1 is remarkably flat over this redshift range. Because the quasar luminosity function is strongly peaked near z ∼ 2, the lack of redshift evolution indicates that star-forming galaxies likely dominate the photoionization rate at z ≥ 3, and possibly at all redshifts probed. A star formation history peaking at redshift z = 5 − 6, as in the theoretical model of Hernquist & Springel, appears favored over the current state-of-the-art observational fit by Hopkins & Beacom, which peaks at z = 2 − 3. The increasing dominance of stellar emission toward high redshifts indicate that galaxies likely reionized the universe.