Metallicity Evolution of Damped Lyman-alpha Systems out to z~5

TL;DR

This study measures the chemical abundances of 47 damped Lyman-alpha systems up to redshift 5, revealing the evolution of cosmic metallicity and its implications for galaxy formation and stellar populations.

Contribution

It provides new metallicity measurements for high-redshift DLAs, extending the metallicity evolution trend to z~5 and analyzing the metallicity distribution and alpha/Fe ratios.

Findings

Metallicity decreases with redshift, with a linear trend of -0.22 dex per unit redshift.

The metallicity 'floor' persists out to z~5, around 1/600 solar.

The scatter in metallicity among DLAs remains ~0.5 dex up to z~5.

Abstract

We present chemical abundance measurements for 47 damped Lyman-alpha systems (DLAs), 30 at z>4, observed with the Echellette Spectrograph and Imager and the High Resolution Echelle Spectrometer on the Keck telescopes. HI column densities of the DLAs are measured with Voigt profile fits to the Lyman-alpha profiles, and we find an increased number of false DLA identifications with SDSS at z>4 due to the increased density of the Lyman-alpha forest. Ionic column densities are determined using the apparent optical depth method, and we combine our new metallicity measurements with 195 from previous surveys to determine the evolution of the cosmic metallicity of neutral gas. We find the metallicity of DLAs decreases with increasing redshift, improving the significance of the trend and extending it to higher redshifts, with a linear fit of -0.22+-0.03 dex per unit redshift from z=0.09-5.06. The metallicity 'floor' of ~1/600 solar continues out to z~5, despite our sensitivity for finding DLAs with much lower metallicities. However, this floor is not statistically different from a steep tail to the distribution. We also find that the intrinsic scatter of metallicity among DLAs of ~0.5 dex continues out to z~5. In addition, the metallicity distribution and the alpha/Fe ratios of z>2 DLAs are consistent with being drawn from the same parent population with those of halo stars. It is therefore possible that the halo stars in the Milky Way formed out of gas that commonly exhibits DLA absorption at z>2.