The Effect of Fluctuations on the Helium-Ionizing Background

Interpretation of He II Lyman 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 of the IGM. Our model results in a He II ionizing background that evolves steeply with redshift, increasing by a factor of >~3.5 from z = 3.5 to z = 2.5. This causes rapid evolution in the mean He II Lyman alpha optical depth -- as recently observed -- without appealing to the reionization of He II. Such 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