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

Abstract

We investigate the implications of our measurement of the Lyα forest opacity at redshifts 2 ~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.