Annu. Rev. Astron. Astrophys. 1991. 29: 239-274
Copyright © 1991 by . All rights reserved

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3.4.3 COLD GAS The kinematics of cold gas provides information on the intrinsic shape (86, 141, 142, 244, 342). If the potential is not circular in the plane of a gas disk, then the gas will be on elliptical orbits and measurements of the surface brightness of the disk and its velocity field constrain the shape of the potential in the plane of the disk. If the gas has settled in a symmetry plane, the position angle offset between the stellar body and the gas disk provides an extra constraint on the shape. Data on NGC 5128 (372), NGC 1052 (69), NGC 4278 (208), NGC 5077 (31), and the bulge of NGC 5845 (33, 143) have been interpreted with gas on elliptic orbits in triaxial models. Most of the models assume that the gas has settled. The validity of this assumption is uncertain (85), as the settling time in the outer parts of galaxies can be rather long (321, 322), and Sparke has shown that self-gravitating rings or disks may never settle (315). Furthermore, the derived intrinsic shapes may not be representative, as the lifetime of the gas disk may depend on the shape of the galaxy.