Abstract.
We have mapped the central region of the spiral galaxy NGC 1808 in the
12CO J=1-0, J=2-1, J=3-2 lines and measured 13CO J=1-0, J=2-1,
C18O J=1-0, HCN J=1-0 and an upper limit on CS J=2-1 at the central
position. We have also measured the 12CO 2-1/1-0 and 12CO/13CO J=1-0
intensity ratios at two positions outside the disk. NGC 1808 has highly
concentrated 12CO emission and an asymetrical velocity field in 12CO
that shows distinct peaks in both sides of the centre. The line intensity
ratios indicate that the central molecular gas of NGC 1808 is in a state
of high excitation compared with giant molecular clouds in the disk of
the Milky Way. We present a ``two-phase'' model that can explain the
observations with a warm Tk = 100 K, diffuse low-mass medium of moderate
density, n(H2) = 2000 cm^-3, surrounding colder Tk = 15 K, dense cores,
n(H2) = 10e5 cm^-3. A simple estimate of the pressure felt by these clouds
leads to further constrains on their properties.
The statistically enhanced 12CO 2-1 map supports the notion of a central
molecular ring, but the 12CO emission line profile shows three peaks, two
peaks at the high velocity (NW) side of the nominal central velocity and
one on the low velocity (SE) side, indicating a somewhat asymmetric
distribution of molecular gas. The 12CO 2-1/1-0 and 12CO/13CO J=1-0
intensity ratios suggest a gradient in the CO excitation in which the
excitation decreases with increasing distance from the nucleus.
Keywords. galaxies: ISM -- galaxies: individual: NGC 1808 -- galaxies: kinematics and dynamics -- radio lines: galaxies