The Effect of Fluctuations on the Helium-Ionizing Background

TL;DR

This paper models the He II ionizing background considering quasar fluctuations and IGM absorbers, explaining rapid optical depth evolution without requiring He II reionization.

Contribution

It introduces a radiative transfer model that incorporates quasar discreteness and background fluctuations, providing new insights into the evolution of the He II ionizing background.

Findings

He II ionization rate increases steeply from z=3.0 to 2.5

Rapid optical depth evolution explained without He II reionization

Model aligns with recent observations of Lyα absorption

Abstract

Interpretation of He II Ly{\alpha} 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 Ly{\alpha} 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.