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 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.

5 pages, including 1 figure, published by ApJ

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• Quasars Are Not Light-Bulbs: Testing Models of Quasar Lifetimes with the Observed Eddington Ratio Distribution
• A downturn in intergalactic CIV as redshift 6 is approached
• An unbiased measurement of the UV background and its evolution via the proximity effect in quasar spectra
• The simulated 21 cm signal during the epoch of reionization : full modeling of the Ly-alpha pumping
• VoIgt profile Parameter Estimation Routine (VIPER): H I photoionization rate at z<0.5
• Probing the Epoch of Reionization with the Lyman Alpha Forest at z~4-5
• The cosmic dust rate across the Universe
• High redshift AGNs and HI reionisation: limits from the unresolved X-ray background
• Constraining the quasar population with the broad-line width distribution
• Cosmological-Scale Lyman-alpha Forest Absorption Around Galaxies and AGN Probed with the HETDEX and SDSS Spectroscopic Data