On Lyman-limit Systems and the Evolution of the Intergalactic Ionizing Background

TL;DR

This paper investigates the properties of Lyman-limit systems and their impact on the intergalactic ionizing background, using simulations and observations to understand their evolution and the ionizing emissivity from redshift 3 to 6.

Contribution

It provides a detailed comparison of simulations with observations, estimates the intergalactic gas clumping factor post-reionization, and explores the sensitivity of the ionizing background to source emissivity changes.

Findings

Simulations reproduce observed Lyman-limit systems abundance.

Small changes in ionizing emissivity cause large background variations.

Clumping factor of intergalactic gas at z=6 estimated as 2-3.

Abstract

We study the properties of self-shielding intergalactic absorption systems and their implications for the ionizing background. We find that cosmological simulations post-processed with detailed radiative transfer calculations generally are able to reproduce the observed abundance of Lyman-limit systems, and we highlight possible discrepancies between the observations and simulations. This comparison tests cosmological simulations at overdensities of ~100. Furthermore, we show that the properties of Lyman-limit systems in these simulations, in simple semi-analytic arguments, and as suggested by recent observations indicate that a small change in the ionizing emissivity of the sources would have resulted in a much larger change in the amplitude of the intergalactic HI-ionizing background (with this scaling strengthening with increasing redshift). This strong scaling could explain the rapid evolution in the Lyman-alpha forest transmission observed at z = 6. Our calculations agree with the suggestion of simpler models that the comoving ionizing emissivity was constant or even increasing from z = 3 to 6. Our calculations also provide a more rigorous estimate than in previous studies for the clumping factor of intergalactic gas after reionization, which we estimate was 2 - 3 at z = 6.