Quasar Absorption-Line Systems and Astrobiology

Abstract

Quasars are among the most powerful background sources for probing gas at cosmological distances by means of absorption-line spectroscopy. Quasar spectra show a large number of absorption lines, most of which are attributed to the Ly α transition of neutral hydrogen (HI) originating in layers of gas located in the direction of the quasar. Thanks to the cosmological expansion of the universe, each Ly α line falls at a different wavelength in the observer’s rest frame according to the redshift of the layer. In most cases, the HI lines are weak and do not show accompanying metal lines at the same redshift. These weak lines are believed to originate in the intergalactic medium, an environment highly ionized and extremely metal poor. Our attention here is focused on a less frequent type of quasar absorbers, characterized by very strong Ly α profiles broadened by radiation damping and called damped Ly α absorptions (Wolfe et al. 1986). These absorptions are always accompanied by a complex of low-ionization metal lines at the same redshift, all together forming a Damped Ly α (DLA) system. There is general agreement that DLA systems originate in the interstellar medium (ISM) of intervening galaxies. DLAs are most easily identified at redshift z > 2, when the Ly α is redshifted to the optical band, but can be detected up to z ~6, the redshift of the most distant quasars. This redshift interval corresponds to an interval of look-back time between ~10 and ~12 billion years ago, according to the current values of the cosmic expansion parameters. Therefore we can say that DLA studies probe the ISM of galaxies observed at very large look-back times. As a consequence, the link between DLA studies and astrobiology is basically the same that exists between ISM studies and astrobiology, with the advantage that DLA observations probe a variety of galaxies, back to the earliest epochs of their evolution.