Associated Hi in Absorbers at High Redshift

WSRT observations have provided a first inventory of the incidence of H i 21 cm line absorption associated with AGN at redshifts up to z=1.0. There is a large range in line depths, from τ = 0.44 to τ ≤ 0.001, and a substantial variety of line profiles, from Gaussians of less than ten km s to more typically a few hundred km s, as well as irregular and multi-peaked absorption, sometimes spanning many hundreds of km s. The chance to detect appreciable H i absorption is greatest in the most compact radio sources, GPSs and CSOs, where it can occur in circumnuclear “disks” or “tori”, as well as in gas enveloping jets and hot spots; inferred densities range at least between 10 cm and 10 cm. But H i absorption occurs also in some CSSs, perhaps associated with jet-cloud interaction regions, and in quasars with a large optical reddening. VLBI observations at the unusual UHF frequencies of redshifted H i 21 cm can give unique “sight”lines into the physics and evolution of young radio sources and their inner galactic medium.