The ESO UVES Advanced Data Products Quasar Sample - VI. Sub-Damped Lyman-$\alpha$ Metallicity Measurements and the Circum-Galactic Medium

TL;DR

This study measures the metallicity of sub-Damped Lyman-alpha absorbers using UVES data, compares them with DLAs, and explores their relation to the circum-galactic medium, revealing potential bimodal metallicity distributions at low redshift.

Contribution

It introduces a new method to derive absorption profile velocity widths and provides a comprehensive metallicity compilation for sub-DLAs and DLAs, highlighting differences in dust and nucleosynthetic histories.

Findings

Tighter correlation between velocity width and metallicity in DLAs than sub-DLAs.

Hints of bimodal [Fe/H] distribution in low-redshift sub-DLAs.

Differences in dust depletion patterns compared to the Milky Way.

Abstract

The Circum-Galactic Medium (CGM) can be probed through the analysis of absorbing systems in the line-of-sight to bright background quasars. We present measurements of the metallicity of a new sample of 15 sub-damped Lyman-$\alpha$ absorbers (sub-DLAs, defined as absorbers with 19.0 < log N(H I) < 20.3) with redshift 0.584 < $\rm z_{abs}$ < 3.104 from the ESO Ultra-Violet Echelle Spectrograph (UVES) Advanced Data Products Quasar Sample (EUADP). We combine these results with other measurements from the literature to produce a compilation of metallicity measurements for 92 sub-DLAs as well as a sample of 362 DLAs. We apply a multi-element analysis to quantify the amount of dust in these two classes of systems. We find that either the element depletion patterns in these systems differ from the Galactic depletion patterns or they have a different nucleosynthetic history than our own Galaxy. We propose a new method to derive the velocity width of absorption profiles, using the modeled Voigt profile features. The correlation between the velocity width delta_V90 of the absorption profile and the metallicity is found to be tighter for DLAs than for sub-DLAs. We report hints of a bimodal distribution in the [Fe/H] metallicity of low redshift (z < 1.25) sub-DLAs, which is unseen at higher redshifts. This feature can be interpreted as a signature from the metal-poor, accreting gas and the metal-rich, outflowing gas, both being traced by sub-DLAs at low redshifts.