The effect of fluctuations on the helium-ionizing background

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 intergalactic medium 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 of similar to 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 HI column density absorbers becomes fully ionized.