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 \tau = 0.44 to \tau \leq 0.001 , and a substantial variety of line profiles , from Gaussians of less than ten km s ^ { -1 } to more typically a few hundred km s ^ { -1 } , as well as irregular and multi-peaked absorption , sometimes spanning many hundreds of km s ^ { -1 } .
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 ^ { -3 } and 10 ^ { 4 } cm ^ { -3 } .
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 .