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The Puzzle

There are mainly two questions to be addressed: 1) What causes the large velocity width (several hundred kms^-1) of the absorption lines detected against the nuclear regions of galaxies? 2) Why are the absorption line widths observed against the nuclear radio continuum similar to those measured in emission over the whole extent of the galaxy? The problem is illustrated well in Fig. 1 which shows a typical HI position-velocity diagram of a starburst galaxy (here NGC 1808); both the HI emission and absorption lines cover nearly 400 kms^-1. To address these question one should first examine the conditions for observing neutral hydrogen gas in absorption³: we need cold atomic hydrogen gas ($T \la$ 150 K) in front of a rather strong continuum source. Then, there are mainly two explanations for the large velocity range observed in absorption. The gas is either A) in turbulent motion or B) in a regular Keplerian orbit around the nucleus. Case A) means there are numerous cold gas clouds falling into and ejected out of the nuclear region. To explain the similar velocity range for emission and absorption the clouds are required to have maximum velocities similar to the rotation amplitude of the galaxy. This model is, e.g., strongly favoured by Mirabel & Sanders (1988) who in their paper conclude that ``... in the central regions of the most luminous infrared galaxies there must be high concentrations of turbulent atomic gas enshrouding the nuclear radio-continuum source.'' Case B) requires rather high rotational velocities close to the nucleus and was therefore often rejected. But now that interferometers and VLBI techniques are used to resolve the nuclear continuum structure in galaxies those high rotation velocities have been found in nearly all cases (see Section 4). Most of the large absorption line widths can be reproduced if the rotation amplitude in the nuclear region is similar to that in the outer region of the galaxy. This requires either a rather flat rotation curve or two components with similar amplitude as for example present in the Galactic rotation curve (Dame et al. 1987). In some cases the inner part of the rotation curve has to rise toward the nucleus to explain absorption line widths much larger than the observed velocity range in emission. The extreme rotational velocities of the nuclear maser emission in NGC 4258 shows what might be happening in the centre of many starburst galaxies (see Section 4.3).

A more detailed discussion of the two cases is given by Koribalski, Dickey & Mebold (1993). In several galaxies both high rotational velocities and either infall or outflow of gas are observed.