This section describes a group of relatively nearby spiral galaxies showing
a variety
of activities. The original selection criterion for this sample was the
presence of broad HI absorption lines -- a quite unusual criterion
at first glance, but a very valuable one in the later
interpretation of the overall kinematics in these galaxies. Such broad
absorption lines are quite peculiar, because they indicate a large range
of velocities both blue- and red-shifted in a very tiny region just in front of the radio continuum emission. Since such a broad HI
absorption line was detected in NGC 1808 (see below) I wanted to know its
origin and the relation to the surrounding medium in the galaxy.
The galaxies with broad HI absorption lines can be divided into
two different groups, one consisting of relatively nearby luminous spirals
(see Tables 1, 2 and 3) and the other of distant ultra-luminous mergers (see
Table 4). In the following I will take a closer look at the members of
the first group.
Starbursts are thought to be the result of gas accretion toward the
central regions of galaxies. There are mainly two mechanisms known which can
transport mass into this region: either tidal interactions or bars (see
e.g., Combes 1988). I suggest that the broad HI absorption lines
in the central region of most luminous spirals (see Table 1) are caused by
a fast-rotating ring of cold gas. The accretion of gas in the inner region
(probably near the inner Lindblad resonances) could have been induced by
the bar potential of these galaxies (see e.g., Combes & Gerin 1985). Since
outflow of gas from the central region is another phenomenon observed in
this group of galaxies I propose a model in which the neutral gas is
fuelling the nuclear starburst where it is partly ionized and then ejected
by supernova explosions and stellar winds (see Section 5).
