Evolution of the Intergalactic Opacity: Implications for the Ionizing Background, Cosmic Star Formation, and Quasar Activity

Abstract

We investigate the implications of the intergalactic opacity for the evolution of the cosmic UV luminosity density and its sources. Our main constraint is our measurement of the Lya forest opacity at redshifts 2<z<4.2 from 86 high-resolution quasar spectra. In addition, we impose the requirements that HI must be reionized by z=6 and HeII by z~3, and consider estimates of the hardness of the ionizing background from HI to HeII column density ratios. The derived hydrogen photoionization rate is remarkably flat over the Lya forest redshift range covered. 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. Combined with direct measurements of the galaxy UV luminosity function, this requires only a small fraction f_esc~0.5% of galactic hydrogen ionizing photons to escape their source for galaxies to solely account for the entire ionizing background. Under the assumption that the galactic UV emissivity traces the star formation rate, current state-of-the-art observational estimates of the star formation rate density appear to underestimate the total photoionization rate at z~4 by a factor ~4, are in tension with recent determinations of the UV luminosity function, and fail to reionize the Universe by z~6 if extrapolated to arbitrarily high redshift. A theoretical star formation history peaking earlier fits the Lya forest photoionization rate well, reionizes the Universe in time, and is in better agreement with the rate of z~4 gamma-ray bursts observed by Swift. Quasars suffice to doubly ionize helium by z~3 and likely contribute a non-negligible and perhaps dominant fraction of the hydrogen ionizing background at their z~2 peak. [Abridged]

26 pages, including 14 figures, submitted to ApJ and revised following the referee's report

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• Measuring the temperature and profiles of Lyman-$\alpha$ absorbers
• Gamma-Ray Burst Luminosity Functions Based On a Newly Discovered Correlation Between Peak Spectral Energy and V/V_max
• Evidence of bimodal physical properties of intervening, optically-thin CIII absorbers at z ~ 2.5
• The HeI 584 A Forest as a Diagnostic of Helium Reionization
• Spatial Correlations in the Helium-Ionizing Background
• Probing patchy reionisation with JWST: IGM opacity constraints from the Lyman-$\alpha$ forest of galaxies in legacy extragalactic fields
• Triple-ionised carbon associated with the low-density neutral hydrogen gas at 1.7 < z < 3.3: the integrated N(HI)-N(CIV) relation
• Measuring the thermal and ionization state of the low-$z$ IGM using likelihood free inference
• Modeling the Intergalactic Medium during the Epoch of Reionization
• Candidate z ~ 2.5 Lyman Continuum Sources in the GOODS Fields
• A Novel Estimator for the Equation of State of the IGM by Ly$\alpha$ Forest Tomography
• Boosting Line Intensity Map Signal-to-Noise with the Ly-$\alpha$ Forest Cross-Correlation
• Probing Large Scale Ionizing Background Fluctuation with Lyman $\alpha$ Forest and Galaxy Cross-correlation at z=2.4
• Trend Filtering -- II. Denoising Astronomical Signals with Varying Degrees of Smoothness
• HeII Ly$\alpha$ Transmission Spikes and Absorption Troughs in Eight High-resolution Spectra Probing the End of HeII Reionization
• Large scale environmental bias of the QSO line of sight proximity effect
• On the unique evolutionary mechanisms of massive quiescent galaxies in the epoch of reionisation
• Origin of Metals around Galaxies I: Catalogs of Metal-line Absorption Doublets from High-Resolution Quasar Spectra
• Estimation of the temperature-density relation in the intergalactic medium at $\mathbf{z}\sim 2-4$ via Ly$\alpha$ forest
• Jeans smoothing of the Ly$\alpha$ forest absorption lines
• Rise of the forsaken relics: connecting present-day stellar streams and phase-mixed galaxies to the Epoch of Reionization
• Equation of state of the intergalactic medium in the early Universe
• A quantitative assessment of completeness correction methods and public release of a versatile simulation code