Observations of HI absorbing gas in compact radio sources at cosmological redshifts

We present an overview of the occurrence and properties of atomic gas associated with compact radio sources at redshifts up to z = 0.85. Searches for H I 21 cm absorption were made with the Westerbork Synthesis Radio Telescope at UHF-high frequencies (725 - 1200 MHz). Detections were obtained for 19 of the 57 sources with usable spectra (33%). We have found a large range in line depths, from tau = 0.16 to tau less than or equal to 0.001. There is a substantial variety of line profiles, including Gaussians of less than 10 km s(-1), to more typically 150 km s(-1), as well as irregular and multi-peaked absorption profiles, sometimes spanning several hundred km s(-1). Assuming uniform coverage of the entire radio source, we obtain column depths of atomic gas between 1 x 10(19) and 3.3 x 10(21) (T-sp/100 K)(1/f) cm(-2). There is evidence for significant gas motions, but in contrast to earlier results at low redshift, there are many sources in which the H I velocity is substantially negative (up to v = - 1420 km s(-1)) with respect to the optical redshift, suggesting that in these sources the atomic gas, rather than falling into the centre, may be be flowing out, interacting with the jets, or rotating around the nucleus.