Speaker
Mr
Frederick Davies
(UCLA)
Description
The standard cosmological radiative transfer method used to estimate the He II ionizing background assumes a uniform radiation field despite the discrete nature of the (rare) bright quasars that dominate the background. We 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, which causes rapid evolution in the mean He II optical depth -- as recently observed -- without appealing to the reionization of He II. The observed behavior could instead result from rapid evolution in the mean free path of ionizing photons as the helium in higher column density H I absorbers becomes fully ionized.
