Ca II and Na I Quasar Absorption-Line Systems in an Emission-Selected Sample of SDSS DR7 Galaxy/Quasar Projections: I. Sample Selection

TL;DR

This study identifies low-redshift galaxy hosts of quasar absorption-line systems using SDSS data, finding 4 high-confidence systems associated with specific galaxy types, and providing a valuable catalog for future research.

Contribution

The paper introduces a method to select low-redshift galaxy hosts of absorption systems and presents a catalog of 108 systems, including 4 high-confidence galaxy-associated cases, from SDSS DR7 data.

Findings

Most absorption systems are from Galactic disk, halo, or Virgo cluster sightlines.

Identified 4 high-confidence galaxy-associated absorption systems.

Catalog of 108 absorption systems provided for future studies.

Abstract

The aim of this project is to identify low-redshift host galaxies of quasar absorption-line systems by selecting galaxies which are seen in projection onto quasar sightlines. To this end, we use the Seventh Data Release of the Sloan Digital Sky Survey (SDSS-DR7) to construct a parent sample of 97489 galaxy/quasar projections at impact parameters of up to 100 kpc to the foreground galaxy. We then search the quasar spectra for absorption line systems of Ca II and Na I within +- 500 km/s of the galaxy's velocity. This yields 92 Ca II and 16 Na I absorption systems. We find that most of the Ca II and Na I systems are sightlines through the Galactic disk, through High Velocity Cloud complexes in our halo, or Virgo cluster sightlines. Placing constraints on the absorption line rest equivalent width significance (>=3.0 sigma), the Local Standard of Rest velocity along the sightline (>= 345 km/s), and the ratio of the impact parameter to the galaxy optical radius (<=5.0), we identify 4 absorption line systems that are associated with low-redshift galaxies at high confidence, consisting of two Ca II systems (one of which also shows Na I), and two Na I systems. These 4 systems arise in blue, L_r^* galaxies. Tables of the 108 absorption systems are provided to facilitate future follow up.