ar X iv : a st ro - p h / 98 06 18 2 v 1 1 2 Ju n 19 98 Redshifted 21 cm Line Absorption by Intervening Galaxies

The present generation of radio telescopes, combined with powerful new spectrometers, is opening a new age of redshifted radio absorption-line studies. Outfitting of arrays of antennas, such as the European VLBI Network and the upgraded VLA, with flexibly tuned receivers, will measure sizes and kinematics of intervening galaxies as a function of cosmic time. Shortly after the discovery that quasars were highly luminous objects located at cosmological distances, Wagoner (1967), Bahcall & Spitzer (1969), and Gunn & Peterson (1965) recognized the importance of absorption arising in intervening galaxies, extended halos and intergalactic matter, along with use of the 21cm line as a probe of neutral gas (Bahcall and Ekers 1969). Quasar absorption lines have subsequently proven to be a gold mine of information on the evolution of the gaseous content of the Universe. The following subsection is a reminder that the 21cm line techniques can address the epoch during which mass is being most vigorously redistributed and galaxies are being assembled. A detectable 21cm line optical depth, τ 21 , requires a high column density of neutral gas, N HI ≈ 2×10 21 ∆V 10 km/s T s 100 K τ 21 cm −2. For the ranges of spin temperatures T s and velocity widths ∆V that are typical of quasar absorption line systems (see Lane these proceedings), N HI are of order 10 21 cm −2 or more. Gas clouds of such high column density seen in absorption against quasars are drawn from the extreme high N HI end of the column density distribution (cf Petitjean et al 1993) identified through absorption in the ultraviolet Lyman-α line. The high column densities of N HI > 10 20 cm −2 , which are typical of lines of sight through the cool disks of nearby late-type galaxies, are named damped Lyman-α systems (DLa's) after their distinctive absorption profile; these systems form the dominant repositories of neutral gas detected by the absorption-line method (Wolfe et al 1995, Lanzetta et al 1995). The exact nature of these absorption systems remains to be determined, but it is clear that, by tracing the confining gravitational potentials that are necessary keep 1 hydrogen neutral in the face of ionizing background radiation, they can sense the evolution of galaxies as a function of redshift. Optical spectroscopy has been the most efficient method for identifying the DLa's, which make interesting targets for 21cm absorption against background radio quasars. …