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

TL;DR

This study shows that the hydrogen photoionization rate remains flat between redshifts 2 and 4.2, suggesting star-forming galaxies, rather than quasars, dominated cosmic reionization and star formation did not decline beyond z~3.

Contribution

It provides new measurements of the Lyman-alpha forest opacity indicating a flat ionization rate, supporting galaxy-driven reionization and challenging previous star formation decline models.

Findings

Hydrogen photoionization rate is flat from z=2 to 4.2.

Star-forming galaxies likely dominate ionizing emissivity at z>3.

Star formation rate density does not decline beyond z~3.

Abstract

We investigate the implications of our measurement of the Lyman-alpha 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 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.