Probing Structure in Cold Gas at $z \lesssim 1$ with Gravitationally Lensed Quasar Sight Lines

TL;DR

This study uses gravitationally lensed quasars to investigate the structure and metallicity gradients of cold gas in foreground galaxies at redshifts below 1, revealing complex absorption features and potential signs of galaxy interactions.

Contribution

It provides new measurements of H I and metal abundances along multiple sight lines in lens galaxies, doubling the existing sample and offering insights into their metallicity gradients and mass-metallicity relation.

Findings

No strong correlation between absorption differences and sight line separation.

Tentative anti-correlation between abundance gradients and galaxy center metallicities.

Some galaxies show inverted metallicity gradients, indicating possible merger activity.

Abstract

Absorption spectroscopy of gravitationally lensed quasars (GLQs) enables study of spatial variations in the interstellar and/or circumgalactic medium of foreground galaxies. We report observations of 4 GLQs, each with two images separated by 0.8-3.0", that show strong absorbers at redshifts 0.4$<$$z_{abs}$$<$1.3 in their spectra, including some at the lens redshift with impact parameters 1.5-6.9 kpc. We measure H I Lyman lines along two sight lines each in five absorbers (10 sight lines in total) using HST STIS, and metal lines using Magellan Echellette or Sloan Digital Sky Survey. Our data have doubled the lens galaxy sample with measurements of H I column densities ($N_{\rm H I}$) and metal abundances along multiple sight lines. Our data, combined with the literature, show no strong correlation between absolute values of differences in $N_{\rm H I}$, $N_{\rm Fe II}$, or [Fe/H] and the sight line separations at the absorber redshifts for separations of 0-8 kpc. The estimated abundance gradients show a tentative anti-correlation with abundances at galaxy centers. Some lens galaxies show inverted gradients, possibly suggesting central dilution by mergers or infall of metal-poor gas. [Fe/H] measurements and masses estimated from GLQ astrometry suggest the lens galaxies lie below the total mass-metallicity relation for early-type galaxies as well as measurements for quasar-galaxy pairs and gravitationally lensed galaxies at comparable redshifts. This difference may arise in part from the dust depletion of Fe. Higher resolution measurements of H and metals (especially undepleted elements) for more GLQ absorbers and accurate lens redshifts are needed to confirm these trends.