NGC 1808 is a peculiar southern spiral galaxy and rather
nearby example of nuclear starburst activity
(
L); for a comprehensive overview see Koribalski (1993).
Well known are the prominent dust filaments which emerge nearly perpendicular
to its central disk (Véron-Cetty & Véron 1985; Laustsen, Madsen & West
1987), similar to those observed in M 82, indicating continuing disk-halo
interactions.
The nuclear region contains a number of bright `hot spots', which are mostly
interpreted as very bright HII regions (Sérsic & Pastoriza 1965;
Alloin & Kunth 1979; Koribalski & Dettmar 1993).
In the actual nucleus, however, Véron-Cetty & Véron
drew attention to the presence of a broad Seyfert-like component of H
with FWHM about 550 kms-1. Radio observations at 6 cm reveal a number
of very small (<1) compact sources in the central region, most
likely to be supernovae and supernova remnants (Saikia et al. 1990).
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.
Figure 5:
HI distribution of the starburst galaxy NGC 1808.
Figure 6:
HI mean velocity field of NGC 1808 (for a full
description see Koribalski 1993). These data were taken with the BnC
and AnB arrays of the VLA (5 h each).