Large-Scale Fluctuations in the HeII Lyman-alpha Forest and HeII Reionization

Abstract

We examine large-scale fluctuations in the HeII Lyman-alpha 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 (~10-100 Mpc across). We compare our models to existing data along five HeII Lyman-alpha 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 ~30 Mpc scales observed at z~2.7-2.9, and the measured optical depths (tau>4) at z>2.9 are difficult to explain with a smoothly-evolving mean radiation field. In order to better understand this 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.

11 pages, 4 figures, submitted to ApJ

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• New Evidence for Extended HeII Reionization at z>3.5 from HeII Lyman Alpha and Beta Transmission Spikes
• Constraints on Quasar Lifetimes and Beaming from the HeII Lyman-alpha Forest
• HST Spectral Observations near the He II Ly-alpha Break: Implications for He II Reionization
• Constraining the Contribution of Galaxies and Active Galactic Nuclei to Cosmic Reionization
• He II Lyβ Gunn-Peterson Absorption: New HST Observations, and Theoretical Expectations
• Semi-numeric simulations of helium reionization and the fluctuating radiation background
• Eddington accreting Black Holes in the Epoch of Reionization
• The impact of faint AGN discovered by JWST on reionization
• The HeI 584 A Forest as a Diagnostic of Helium Reionization
• Cosmic reionization of hydrogen and helium: contribution from both mini-quasars and stars
• HeII Ly$\alpha$ Transmission Spikes and Absorption Troughs in Eight High-resolution Spectra Probing the End of HeII Reionization