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The best tracer for the overall gas distribution and kinematics in galaxies is the 21-cm emission line of the neutral hydrogen atom (HI). Although the HI content and global spectrum of a galaxy can be measured with a single-dish telescope, for this project interferometers such as the Australia Telescope Compact Array (ATCA)1in the Southern Hemisphere and the Very Large Array (VLA)2in the Northern Hemisphere are required to map galaxies in detail. For the brightest galaxies we can also use the HI absorption line to study the gas motions close to the centre by using the largest configurations of the existing interferometers. This method has several advantages: 1) at high angular resolution the HI emission is resolved out leaving absorption only (see Dickey 1979), 2) the HI absorption unambiguously traces the gas in front of the continuum sources, and 3) the velocity resolution is usually very high (a few kms-1), much higher than in the studies of optical emission lines. The obvious disadvantage is that absorption studies are limited to bright galaxies, mostly starburst or otherwise active galaxies. And even in those galaxies one has to keep in mind that the absorption depends directly on the distribution and strength of the continuum sources in the nuclear region.
A description of one of the first interferometric studies of HI absorption lines is given by Clark, Radhakrishnan & Wilson (1962) and Clark (1965).
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