The ionizing background at the end of overlap

TL;DR

This paper challenges the traditional view that the overlap phase causes a rapid increase in the ionizing background during reionization, emphasizing the role of Lyman-limit systems and attenuation length evolution.

Contribution

The authors develop an analytic model incorporating Lyman-limit systems to show that overlap alone does not trigger rapid background changes, redefining the understanding of reionization signatures.

Findings

Overlap does not cause a rapid increase in the ionizing background.

Lyman-limit systems and attenuation length evolution are key to background changes.

Semi-numeric models support the analytic results.

Abstract

One of the most sought-after signatures of reionization is a rapid increase in the ionizing background (usually measured through the Lyman-alpha optical depth toward distant quasars). Conventional wisdom associates this with the "overlap" phase when ionized bubbles merge, allowing each source to affect a much larger volume. We argue that this picture fails to describe the transition to the post-overlap Universe, where Lyman-limit systems absorb ionizing photons over moderate lengthscales (20-100 Mpc). Using an analytic model, we compute the probability distribution of the amplitude of the ionizing background throughout reionization, including both discrete ionized bubbles and Lyman-limit systems (parameterized by an attenuation length). We show that overlap does not by itself cause a rapid increase in the ionizing background or a rapid decrease in the mean Lyman-alpha transmission toward distant quasars. More detailed semi-numeric models support these conclusions. We argue that rapid changes should instead be interpreted as evolution in the attenuation length itself, which may or may not be directly related to overlap.