Large fluctuations in the hydrogen-ionizing background and mean free path following the epoch of reionization

Extremely large opaque troughs in the Ly alpha forest have been interpreted as a sign of an extended reionization process below z similar to 6. Such features are impossible to reproduce with simple models of the intergalactic ionizing background that assume a uniform mean free path of ionizing photons. We build a self-consistent model of the ionizing background that includes fluctuations in the mean free path due to the varying strength of the ionizing background and large-scale density field. The dominant effect is the suppression of the ionizing background in large-scale voids due to 'self-shielding' by an enhanced number of optically thick absorbers. Our model results in a distribution of 50 Mpc h(-1) Ly alpha forest effective optical depths that significantly improves agreement with the observations at z similar to 5.6. Extrapolation to z similar to 5.4 and 5.8 appears promising, but matching the mean background evolution requires evolution in the absorber population beyond the scope of the present model. We also demonstrate the need for extremely large volumes (> 400 Mpc on a side) to accurately determine the incidence of rare large-scale features in the Ly alpha forest.