Large-scale fluctuations in the HeII Lyα forest and he II reionization

Abstract

We examine large-scale fluctuations in the HeII Lyα forest transmission during and after HeII 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 (∼10-100Mpc across). We compare our models to existing data along five HeII Lyα forest lines of sight spanning z ∼2-3.2. The large cosmic variance contained in our model plausibly explains many of the observed fluctuations at z ≲ 2.5. But our models cannot accommodate the large fluctuations toward high optical depths on 30Mpc scales observed at z ∼2.7-2.9, and the measured optical depths (τeff ≳ 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 HeII 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, HeII reionization does not complete until z ≲ 2.9. © 2010. The American Astronomical Society.