Sub-damped Lyman alpha systems in the XQ-100 survey II -- Chemical evolution at 2.4<z<4.3

TL;DR

This study analyzes 155 sub-damped Lyman alpha systems at redshifts 2.4 to 4.3 to understand their role in cosmic chemical evolution, revealing they are generally more metal-poor than DLAs when ionization effects are considered.

Contribution

First comprehensive metallicity analysis of subDLAs over 2.4<z<4.3 using ionization corrections with MCMC and Cloudy models, highlighting their chemical evolution and relation to DLAs.

Findings

SubDLAs are more metal-poor than DLAs after ionization correction.

Gas-phase [Si/Fe] in subDLAs is similar to DLAs, indicating comparable dust depletion.

[C/O] remains solar across metallicities, suggesting carbon-rich ejecta in circumgalactic environments.

Abstract

We present the measured gas-phase metal column densities in 155 sub-damped Lyman alpha systems (subDLAs) with the aim to investigate the contribution of subDLAs to the chemical evolution of the Universe. The sample was identified within the absorber-blind XQ-100 quasar spectroscopic survey over the redshift range 2.4<=z<=4.3. Using all available column densities of the ionic species investigated (mainly CIV, SiII, MgII, SiIV, AlII, FeII, CII, and OI; in order of decreasing detection frequency), we estimate the ionization-corrected gas-phase metallicity of each system using Markov Chain Monte Carlo techniques to explore a large grid of Cloudy ionization models. Without accounting for ionization and dust depletion effects, we find that the HI-weighted gas-phase metallicity evolution of subDLAs are consistent with damped Lyman alpha systems (DLAs). When ionization corrections are included, subDLAs are systematically more metal-poor than DLAs (between ~0.5 sigma and ~3 sigma significance) by up to ~1.0 dex over the redshift range 3<=z<=4.3. The correlation of gas-phase [Si/Fe] with metallicity in subDLAs appears to be consistent with that of DLAs, suggesting that the two classes of absorbers have a similar relative dust depletion pattern. As previously seen for Lyman limit systems, the gas-phase [C/O] in subDLAs remains constantly solar for all metallicities indicating that both subDLAs and Lyman limit systems could trace carbon-rich ejecta, potentially in circumgalactic environments.