Non-equilibrium ionization in the multiphase circumgalactic medium -- impact on quasar absorption-line analyses

TL;DR

This study develops a time-dependent photoionization framework to better understand ionization conditions in the multiphase circumgalactic medium, revealing significant effects of rapid cooling on ion fractions and elemental abundance in quasar absorption systems.

Contribution

It introduces a self-consistent TDP modeling approach that accounts for rapid cooling, improving upon the standard equilibrium assumptions in analyzing quasar absorption lines.

Findings

TDP ion fractions vary significantly with UVB and initial temperature assumptions.

Gas density estimates are consistent between PIE and TDP models.

Non-solar abundance patterns are robustly constrained from observed ion ratios.

Abstract

This paper presents an updated framework for studying the ionizing conditions and elemental abundances of photoionized, metal-enriched quasar absorption systems. The standard assumption of ionization equilibrium invoked in absorption line analyses requires gas to cool on longer timescales than ionic recombination (t_cool >> t_rec). However, this assumption may not be valid at high metallicities due to enhanced cooling losses. This work presents a suite of time-dependent photoionization (TDP) models that self-consistently solve for the ionization state of rapidly cooling gas irradiated by the extragalactic ultraviolet background (UVB). The updated framework explores various revised UVBs from recent studies, a range of initial temperatures, and different elemental abundance patterns to quantify the effects of TDP on the observed ion fractions. A metal-enriched ($\mathrm{[\alpha/H]}=0.6_{-0.1}^{+0.2}$) C IV absorption system at z ~ 1 previously studied using photoionization equilibrium (PIE) models is re-examined under the TDP framework. The main findings are as follows: (1) varying prescriptions for the underlying UVB or adopting initial temperatures T_0 < 1e6 K (with the starting ionization state in collisional equilibrium) change TDP ion fractions by up to a factor of three and ten respectively, but the adopted relative elemental abundance pattern affects ion fractions by at most 40%; (2) the inferred gas densities are consistent between PIE and TDP, but under TDP solar metallicity cannot be ruled out at more than 2-$\sigma$ significance and a non-solar [C/$\alpha$]$\approx 0.25$ is robustly constrained from the observed relative ion abundances. Extending the TDP analyses to a larger sample of super-solar absorption components with high signal-to-noise absorption spectra is needed to quantify the fraction of metal absorbers originating in rapid cooling gas.