LARGE-SCALE FLUCTUATIONS IN THE He II Lyα FOREST AND He II REIONIZATION

We examine large-scale fluctuations in the He II Ly alpha forest transmission during and after He II reionization. We use a simple Monte Carlo model to distribute quasars throughout a large volume and compute the resulting radiation field along one-dimensional skewers. In agreement with previous studies, we find that the rarity of these sources induces order unity fluctuations in the mean optical depth after reionization, even when averaged over large segments (similar to 10-100 Mpc across). We compare our models to existing data along five He II Ly alpha forest lines of sight spanning z similar to 2-3.2. The large cosmic variance contained in our model plausibly explains many of the observed fluctuations at z less than or similar to 2.5. But our models cannot accommodate the large fluctuations toward high optical depths on similar to 30 Mpc scales observed at z similar to 2.7-2.9, and the measured optical depths (tau(eff) greater than or similar to 4) at z > 2.9 are difficult to explain with a smoothly evolving mean radiation field. In order to better understand these data, we construct a toy model of He II reionization, in which we assume that regions with the smallest radiation fields in a post-reionization universe (or farthest from strong ionizing sources) are completely dark during reionization. The observed fluctuations fit much more comfortably into this model, and we therefore argue that, according to present data, He II reionization does not complete until z less than or similar to 2.9.