The observed ionization rate of the intergalactic medium and the ionizing emissivity at z >5: Evidence for a photon starved and extended epoch of reionization

Abstract

We use a large set of hydrodynamical simulations, combined with measurements of the Lyman alpha opacity of the IGM taken from the literature, to obtain robust estimates for the photoionization rate per hydrogen atom at z=5 and 6. We find the photoionization rate drops by a factor of two and four, respectively, compared to our recent measurements at z = 2 - 4. The number of ionizing photons emitted by known sources at z=5 and 6, based on an extrapolation of source numbers below the detection limit and standard assumptions for the relationship between the ionizing emissivity and observed luminosity density at 1500 Angstroms, are in reasonable agreement with the photoionization rates inferred from the Lyman alpha forest if the escape fraction of ionizing photons from galaxies is large (>= 20 per cent). Claims to the contrary may be attributed to the adoption of an unduly high value for the clumping factor of ionized hydrogen. Using physically motivated assumptions for the mean free path of ionizing photons our measurements of the photoionization rate can be turned into an estimate of the ionizing emissivity. In comoving units the inferred ionizing emissivity is nearly constant over the redshift range 2-6 and corresponds to 1.5-3 photons emitted per hydrogen atom over a time interval corresponding to the age of the Universe at z=6. This strongly suggests that the epoch of reionization was photon-starved and extended. [Abridged]

20 pages, 9 figures, accepted for publication in MNRAS

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• He II optical depth and UV escape fraction of galaxies
• Ionizing photon production and escape fractions during cosmic reionization in the TNG50 simulation
• Fossil Ionized Bubbles Around Dead Quasars During Reionization
• The importance of stochasticity in determining galaxy emissivities and UV LFs during cosmic dawn and reionization
• Chemical constraints on the contribution of Population III stars to cosmic reionization
• Efficient adiabatic hydrodynamical simulations of the high-redshift intergalactic medium
• Clustering of Ly-alpha emitters around luminous quasars at z = 2-3: an alternative probe of reionization on galaxy formation
• The role of Pop III stars and early black holes in the 21cm signal from Cosmic Dawn
• The Effect of Cosmic Magnetic Fields on the Metagalactic Ionization Background inferred from the Lyman-α Forest
• Constraining the epoch of reionization from the observed properties of the high-z Universe
• Magnetogenesis at Cosmic Dawn: Tracing the Origins of Cosmic Magnetic Fields
• Modeling Reionization in a Bursty Universe
• The Flatness and Sudden Evolution of the Intergalactic Ionizing Background
• The Infrared Medium-deep Survey. \Romannum{7}. Faint Quasars at $z\sim 5$ in the ELAIS-N1 Field
• Modelling the star-formation activity and ionizing properties of high-redshift galaxies
• The escape fraction of ionizing photons from high redshift galaxies
• The impact and response of minihalos and the inter-halo medium on cosmic reionization
• Reionization and feedback in overdense regions at high redshift
• Photoevaporation of Minihalos during Cosmic Reionization: Primordial and Metal-Enriched Halos
• SCORCH. III. Analytical Models of Reionization with Varying Clumping Factors
• Testing Galaxy Formation Simulations with Damped Lyman-$\alpha$ Abundance and Metallicity Evolution
• Clumping factors of HII, HeII and HeIII
• High-Redshift Galaxy Surveys and the Reionization of the Universe
• The high-redshift tail of stellar reionization in LCDM is beyond the reach of the low-$\ell$ CMB
• Probing reionization using quasar near-zones at redshift z ~ 6
• The End of an Era - The Population III to Population II Transition and the Near Infrared Background
• AGN jets versus accretion as reionization sources
• z ~ 2.5 - 3 Ionizers in the GOODS-N Field
• Constraining the luminosity function of faint undetected i-dropout galaxies
• Spatial Correlations in the Helium-Ionizing Background
• Observing the redshifted 21 cm signal around a bright QSO at $z\sim 10$
• Upper Bound on the First Star Formation History
• Growth and anisotropy of ionization fronts near high redshift quasars in the MassiveBlack simulation
• Candidate z ~ 2.5 Lyman Continuum Sources in the GOODS Fields
• A Cautionary Tale of LyC Escape Fraction Estimates from High Redshift Galaxies
• Modeling the Intergalactic Medium during the Epoch of Reionization
• Lyman Alpha Emitters and the 21cm Power Spectrum as Probes of Density-Ionization Correlation in the Epoch of Reionization
• Probing Large Scale Ionizing Background Fluctuation with Lyman $\alpha$ Forest and Galaxy Cross-correlation at z=2.4
• Cosmic reionization of hydrogen and helium: contribution from both mini-quasars and stars
• The Lyman-limit photon mean free path at the end of late reionisation in the Sherwood-Relics simulations
• Observations of Ly$\alpha$ Emitters at High Redshift
• MCMC determination of the cosmic UV background at $z\simeq 0$ from H$\alpha$ fluorescence
• Fading Light, Fierce Winds: JWST Snapshot of a Sub-Eddington Quasar at Cosmic Dawn
• Project Lyman
• The Shocking Truth: The small contribution to hydrogen reionization from gravitational infall
• Measurement of redshift-space two- and three-point correlation of Ly$\alpha$ absorbers at $1.7<z<3.5$: Implications on evolution of the physical properties of IGM
• Large scale environmental bias of the QSO line of sight proximity effect
• Delayed Enrichment by Unseen Galaxies: Explaining the Rapid Rise in IGM CIV Absorption from z = 6-5
• Gamma-Ray Absorption By The Cosmic Lyman Continuum From Star-forming Galaxies
• Physical conditions in high$-$z optically thin C III absorbers: Origin of cloud sizes and associated correlations
• Joint Lyα emitters - quasars reionization constraints
• Efficient Modelling of Lyman-α opacity fluctuations during late reionization epoch
• Measuring the IGM correlation length at 5<z<6.1: a fast change at the end of Reionization
• An emulator for the ionizing photon mean free path in ultra-high resolution simulations: the implications of mean free path measurements for the reionization history
• Probing Cosmological Reionization through Radio-interferometric Observations of Neutral Hydrogen
• The JWST early galaxy crisis resolved by a reionization degeneracy