Fast Rotation in the Nuclear Region of the Galaxy NGC 1808

(1993) ApJ 402, L41

Abstract. We have mapped the 21-cm absorption in the nuclear region of the starburst galaxy NGC 1808 using the VLA AnB-hybrid array with a resolution of approx. 5.2" (= 275 pc). We find that the broad (Delta_v approx. 360 km/s) absorption line seen at low spatial resolution is in fact a much narrower line which shifts its center velocity over the face of the source. The velocity center of the absorption changes from about +120 km/s relative to the systemic velocity in the NW to -145 km/s in the SE, following the rotation pattern in the outer regions of the galaxy. This is interpreted as a thick ring (torus) of cold, dense gas with a rotation velocity of about 250 km/s and radius 500 pc. From our data the ring is not expanding or infalling rapidly. We suggest that similar HI-rings are also present in numerous other galaxies (e.g. NGC 660, NGC 3079, NGC 4945, ...) for which the width of the unresolved, nuclear absorption line matches very closely the disk rotation velocity. The gravitational mass required to explain this fast rotation in the center of NGC 1808 is a few times 10^9 solar masses. Apparently the distribution of mass goes as 1/r over at least the range from 500 pc to a few kpc.

Keywords. galaxies: active - galaxies: individual (NGC 1808) - galaxies: ISM - galaxies: kinematics and dynamics.

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