H I Column Densities, Metallicities, and Dust Extinction of Metal-Strong Damped Lya Systems

TL;DR

This study identifies and characterizes metal-strong damped Lyman-alpha systems (MSDLAs) at high redshift, revealing their high metallicity, significant dust extinction, and potential as the most chemically evolved gas-rich galaxies in the early universe.

Contribution

First precise measurements of HI column densities and metallicities for MSDLAs, establishing them as the most metal-rich and dust-extinct high-z gas systems, and linking their properties to galaxy mass.

Findings

MSDLAs have higher NHI than typical DLAs.

MSDLAs exhibit median metallicity [M/H] ~ -0.67 at z~2.

Dust causes measurable reddening of background quasars.

Abstract

With the Blue Channel Spectrograph (BCS) on the MMT telescope, we have obtained spectra to the atmospheric cutoff of quasars previously known to show at least one absorption system at z>1.6 with very strong metal lines (candidate metal-strong damped Lya systems; cMSDLAs). The BCS/MMT spectra yield precise estimates of the HI column densities (NHI) of the systems through Voigt profile analysis of their Lya transitions. Nearly all of the cMSDLAs (41/43) satisfy the NHI criterion of DLAs, 10^20.3. As a population, these systems have systematically higher NHI values than DLAs chosen randomly from quasar sightlines. Combining our NHI measurements with previously measured metal column densities, we estimate metallicities for the MSDLAs. These systems have significantly higher values than randomly selected DLAs; at z~2, the MSDLAs show a median metallicity [M/H] ~ -0.67 that is 0.6dex higher than a corresponding control sample. This establishes MSDLAs as having amongst the most metal-rich gas in the high z universe. Our measurements extend the observed correlation between SiII 1526 equivalent width and the gas metallicity to higher values. If interpreted as a mass-metallicity relation, this implies the MSDLAs are the high mass subset of the DLA population. We demonstrate that dust in the MSDLAs reddens their background quasars, with a median shift in the spectral slope of Da = 0.29. Assuming an SMC extinction law, this implies a median reddening E(B-V)=0.025mag and visual extinction A_V=0.076mag. Future studies of MSDLAs offer the opportunity to study the extinction, nucleosynthesis, and kinematics of the most chemically evolved, gas-rich galaxies at high z. [abridged]