E-XQR-30: Evidence for an Increase in the Ionization State of Metal Absorbers from z~6 to z~2

TL;DR

This study analyzes the evolution of metal ionization states in the universe from redshift 6 to 2, revealing a significant increase in ionization levels likely driven by cosmic reionization and changing gas properties.

Contribution

It provides the first comprehensive analysis of ionization state evolution in metal absorbers across this redshift range using a large sample and combines observations with photoionization models.

Findings

Low-ionization systems decrease from 24% to 2% from z~6 to 4.3.

High-ionization systems increase from 52% to 82% over the same redshift range.

The ionization ratio log(N_CII/N_CIV) declines by a factor of ~20 from z~6 to 2.

Abstract

We investigate the evolution of the ionization state of metal-enriched gas in and around galaxies near the epoch of reionization using a sample of 488 metal absorption systems at 4.3<z<6.3 from the E-XQR-30 survey. We classify the absorption systems based on whether they display only low-ionization absorption (CII, SiII, MgII), only high-ionization absorption (CIV, SiIV), or both. The percentage of low-ionization-only systems decreases from 24% at $z\sim$6 to 2% at $z\sim$4.3, whilst the fraction of high-ionization-only systems increases from 52% to 82%. For mixed absorbers (with both low and high ionization absorption), we use the column density ratios log(N_CII/N_CIV) and log(N_SiII/N_SiIV) to quantify the average ionization as a function of redshift. The log(N_SiII/N_SiIV) ratio does not change significantly over 5$\lesssim z \lesssim$6.3. We combine the E-XQR-30 log(N_CII/N_CIV) measurements with literature measurements at $z\sim$2-4 and find that the log(N_CII/N_CIV) ratio declines by a factor of $\sim$20 between $z\sim$6 and $z\sim$2. To explore possible drivers of this evolution, we run photoionization models of gas slabs illuminated by a uniform UV background at fixed density, metallicity and HI column density. We find that the increase in the ionization state of metal absorbers towards lower redshifts can likely be explained by some combination of 1) an increase in the metallicity of CIV-absorbing gas and 2) a decrease in the typical HI column densities of the absorbing gas, driven by the declining cosmic mean density and a rapid rise in the strength of the UV background during the final stages of reionization.