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 <= 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 zgtrsim 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 2008
- DOI:
- 10.1086/590409
- arXiv:
- arXiv:0806.0372
- Bibcode:
- 2008ApJ...682L...9F
- Keywords:
-
- diffuse radiation;
- galaxies: evolution;
- galaxies: formation;
- galaxies: high-redshift;
- methods: data analysis;
- quasars: absorption lines;
- Astrophysics
- E-Print:
- 5 pages, including 1 figure, published by ApJ