Redshifted H I 21 cm line observations of damped LY alpha absorption systems

We have used the Westerbork Synthesis Radio Telescope to search for HI 21 cm absorption associated with high redshift damped Ly alpha systems. We calculate limits to harmonic mean spin temperatures, [T-s], for these systems, and find typical lower limits approximate to 10(3) K, consistent with previous results for other damped Ly alpha systems. We then analyze the relationship between the total HI column density and the integrated 21 cm absorption line equivalent width, EW(21), for damped Ly alpha systems. We compare these results with similar results for lines of sight through our own galaxy. A trend is seen in which the damped Ly alpha systems show systematically lower EW(21) values at a given total N(HI) than Galactic lines of sight, by a factor approximate to 3-5. In the context of a two-temperature-phase model for the absorbing gas, these results suggest that for a given total neutral hydrogen column density, the damped Ly alpha systems contain a larger percentage of warm gas than is seen in typical Galactic lines of sight. We derive upper limits to H I 21 cm emission from the vicinity of the damped Ly alpha absorbers. The mass limits are adequate to ruleout a substantial contribution by protoclusters of the type hypothesized by Oort [A&A, 139, 211 (1984)] to the observed statistics of damped Ly alpha systems at high redshift. Lastly, we derive a lower limit to the molecular hydrogen fraction in the cold gas phase for the absorption system towards 0528-2505 of: F = 2N(H-2)/N(HI)>2%. (C) 1996 American Astronomical Society.