Quasar Absorption Lines and High Redshift Galaxies

Abstract

If metal-line systems occur only where the line of sight to a QSO intercepts a galaxy, we already have information on many hundreds of galaxies at redshifts greater than unity. Here we review research on the connection between galaxies and absorption line systems seen in QSO spectra. Most information about absorption due to galaxies is confined to low-ionization species because strong high-ionization transitions are limited to the vacuum UV. Conversely, the bulk of information about QSO metal-line systems comes from redshifts where galaxies are difficult to observe and may be, due to evolution, very different from present day galaxies. Studies underway to bridge this gap include comparisons of the velocity structure of individual absorbers and galaxies and their clustering, both spatial and in velocity, as well as the galaxian environment of QSO absorbers, and the composition and ionization state of the gas in QSO absorption systems versus galactic coronal clouds. These tend to indicate differences between the properties of high-z QSO absorbers and low-z galaxies, suggesting recent evolution in the population causing the absorption.