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

TL;DR

This study investigates large-scale fluctuations in the HeII Lyman-alpha forest during and after HeII reionization, using models to explain observed optical depth variations and suggesting reionization completes after z<2.9.

Contribution

The paper introduces a simple Monte Carlo model to simulate HeII reionization fluctuations and proposes a toy model indicating reionization likely ends after z<2.9.

Findings

Order unity fluctuations in optical depth post-reionization due to source rarity

Models explain fluctuations at z<2.5 but not at higher redshifts

Reionization probably completes after z<2.9

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.