The Qz5 Survey (II): Metallicity Evolution of Damped Ly{\alpha} Systems Out to z$\sim$5

TL;DR

This study investigates the metallicity evolution of neutral hydrogen gas in the universe using a new sample of high-redshift DLAs, revealing a sharp decrease in metallicity around redshift 5.

Contribution

Introduces a new unbiased sample of DLAs at z > 4.7 and measures the metallicity evolution of HI gas up to z~5, highlighting a significant decline at high redshift.

Findings

Metallicity sharply decreases at z~5.

The average metallicity at z~5 is -2.00, significantly lower than at lower redshifts.

The metallicity evolution slope is -0.22 dex per unit redshift.

Abstract

Damped Ly$\alpha$ absorbers (DLAs) are the highest \HI\ column density (\NHI) absorption line systems detected in the spectra of background quasars. DLAs dominate the neutral gas content of the Universe ($\Omega_{\rm HI}$) and are used to measure the metallicity evolution of \HI\ gas. In this work, we introduce a sample of five recently detected DLAs at $z > 4.7$, found in mid to high-resolution spectroscopy from VLT/X-shooter and Keck/HIRES. These DLAs were not pre-selected based on metallicity, enabling an unbiased study of the metallicity of HI gas at $z \sim 5$. We also search for DLAs unbiased in metallicity at $0<z<5.5$ from the literature, we apply a combined correction for dust depletion and $\alpha$-enhancement (assuming no depletion of S, Si, and Zn) to Fe abundances, and we measure the cosmic metallicity evolution of $\alpha$-elements using a linear fit with a slope of$-0.22 \pm 0.05$ dex per unit redshift. For the highest redshift bin, we find an \NHI weighted average of $\langle Z \rangle = -2.00$. This value is $4.4\sigma$ deviant from the trend recovered at $z < 4.7$ and a K-S test comparison gives a $2.4 \sigma$ difference. We conclude that the metallicity of HI gas sharply decreases at $z \sim 5$, in agreement with previous tentative evidence. This sharp decrease may be connected with the onset of the enrichment of galaxies' circumgalactic media or with the end of cosmic reionization, though we cannot exclude that it is driven by small sample statistics.