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VLA measurements of its nuclear region reveal that the broad (
360 kms-1) HI absorption line seen at low angular
resolution
is a much narrower line which shifts its centre velocity over the face of the
continuum (Koribalski, Dickey & Mebold 1993). This is interpreted as a torus
of cold, dense gas with a rotation velocity of
250 kms-1 and
radius 500 pc. The gravitational mass required to explain this fast rotation
is a few times 109M
.
Figures 5 and 6 display the overall HI distribution and
velocity field of the galaxy NGC 1808. The central `hole' is caused by
HI absorption.
The radius and velocity of the nuclear ring are comparable to the distribution and kinematics of the optically visible `hot spots'. The possibility of a molecular gas ring has been considered by Dahlem et al. (1990) and is also indicated in new CO(2-1) data by Aalto et al. (1994).
A stellar bar (length 3 or 6 kpc), which was discovered in the
H
line (Phillips 1993; Koribalski & Dettmar 1993), might be causing
the gas to accumulate at the inner and outer Lindblad resonances.
The inner Lindblad resonance as determined from the bar pattern speed coincides
with the ring radius; the outer resonance lies just outside the pseudo-ring
which is formed by the outer spiral arms.