The Qz5 Survey. II. Metallicity Evolution of Damped Ly α Systems Out to z  ∼ 5

Damped Ly α absorbers (DLAs) are the highest H i column density ( N H I ) absorption line systems detected in the spectra of background quasars. DLAs dominate the neutral gas content of the Universe (Ω HI ) and are used to measure the metallicity evolution of H i 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 preselected based on metallicity, enabling an unbiased study of the metallicity of H I gas at z ∼ 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 α -enhancement (assuming no depletion of S, Si, and Zn) to Fe abundances, and we measure the cosmic metallicity evolution of α -elements using a linear fit with a slope of −0.22 ± 0.05 dex per unit redshift. For the highest redshift bin, we find an N H I weighted average of 〈 Z 〉 = −2.00. This value is 4.4 σ deviant from the trend recovered at z < 4.7 and a K-S test comparison gives a 2.4 σ difference. We conclude that the metallicity of H I gas sharply decreases at z ∼ 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.