The Flatness and Sudden Evolution of the Intergalactic Ionizing Background

TL;DR

This paper presents a halo-based model explaining the observed flat evolution and rapid decline of the intergalactic ionizing background from redshift 2 to 6, reconciling previous discrepancies between theory and observations.

Contribution

The study introduces a model linking absorbers and sources within halos, explaining the flat ionizing background evolution and its sudden decline at high redshift.

Findings

Model reproduces observed flatness of ionizing background from z~2-5.

Explains rapid decline of ionizing background at z~6.

Shows halo growth influences star formation and opacity over cosmic time.

Abstract

The ionizing background of cosmic hydrogen is an important probe of the sources and absorbers of ionizing radiation in the post-reionization universe. Previous studies show that the ionization rate should be very sensitive to changes in the source population: as the emissivity rises, absorbers shrink in size, increasing the ionizing mean free path and, hence, the ionizing background. By contrast, observations of the ionizing background find a very flat evolution from z~2-5, before falling precipitously at z~6. We resolve this puzzling discrepancy by pointing out that, at z~2-5, optically thick absorbers are associated with the same collapsed halos that host ionizing sources. Thus, an increasing abundance of galaxies is compensated for by a corresponding increase in the absorber population, which moderates the instability in the ionizing background. However, by z~5-6, gas outside of halos dominates the absorption, the coupling between sources and absorbers is lost, and the ionizing background evolves rapidly. Our halo based model reproduces observations of the ionizing background, its flatness and sudden decline, as well as the redshift evolution of the ionizing mean free path. Our work suggests that, through much of their history, both star formation and photoelectric opacity in the universe track halo growth.