The Effect of Fluctuations on the Helium-Ionizing Background

Abstract

Interpretation of He II Lyα absorption spectra after the epoch of He II reionization requires knowledge of the He II ionizing background. While past work has modelled the evolution of the average background, the standard cosmological radiative transfer technique assumes a uniform radiation field despite the discrete nature of the (rare) bright quasars that dominate the background. We implement a cosmological radiative transfer model that includes the most recent constraints on the ionizing spectra and luminosity function of quasars and the distribution of IGM absorbers. We also estimate, for the first time, the effects of fluctuations on the evolving continuum opacity in two ways: by incorporating the complete distribution of ionizing background amplitudes into the standard approach, and by explicitly treating the quasars as discrete -- but isolated -- sources. Our model results in a He II ionization rate that evolves steeply with redshift, increasing by a factor ~2 from z=3.0 to z=2.5. This causes rapid evolution in the mean He II Lyα optical depth -- as recently observed -- without appealing to the reionization of He II. The observed behaviour could instead result from rapid evolution in the mean free path of ionizing photons as the helium in higher H I column density absorbers becomes fully ionized.

14 pages, 9 figures. Accepted by MNRAS; significantly modified from previous version

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Cited by in corpus (24)

• The Evolution of the Intergalactic Medium
• Conditions for Reionizing the Universe with A Low Galaxy Ionizing Photon Escape Fraction
• Evolution of the AGN UV luminosity function from redshift 7.5
• Consistent modelling of the meta-galactic UV background and the thermal/ionization history of the intergalactic medium
• Early and Extended Helium Reionization Over More Than 600 Million Years of Cosmic Time
• The photoheating of the intergalactic medium in synthesis models of the UV background
• Self-Consistent Modeling of Reionization in Cosmological Hydrodynamical Simulations
• Decoding the X-ray Properties of Pre-Reionization Era Sources
• Statistical Detection of the HeII Transverse Proximity Effect: Evidence for Sustained Quasar Activity for $>$25 Million Years
• The Evolution of the HeII-Ionizing Background at Redshifts 2.3<z<3.8 Inferred from a Statistical Sample of 24 HST/COS HeII Ly$\alpha$ Absorption Spectra
• Constraints on QSO emissivity using H I and He II Lyman alpha forest
• A self-consistent 3D model of fluctuations in the helium-ionizing background
• The case against large intensity fluctuations in the z ~ 2.5 HeII Lyman-alpha forest
• Examining the Decline in the C IV Content of the Universe over 4.3 < z < 6.3 using the E-XQR-30 Sample
• Dating individual quasars with the HeII proximity effect
• He II optical depth and UV escape fraction of galaxies
• The impact of temperature fluctuations on the large-scale clustering of the Ly$\alpha$ forest
• Semi-numeric simulations of helium reionization and the fluctuating radiation background
• Probing Inhomogeneity in the Helium Ionizing UV Background
• Probing Large-scale UV Background Inhomogeneity Associated with Quasars using Metal Absorption
• UV background fluctuations traced by metal ions at $z\approx 3$
• Ultraviolet background fluctuations with clustered sources
• Simulating the epoch of Helium Reionization in photon-conserving semi-numerical code SCRIPT
• Measuring photo-ionization rate and mean free path of HeII ionizing photons at $2.5\le z\le 3.6$: Evidence for late and rapid HeII reionization Part-II