The ionizing background at the end of overlap

Understand

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).