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5.3. Results from Gas Rotation Measurements

Generally, elliptical galaxies do not contain cold gas disks or rings that can be used to measure their halo circular velocities, but there are some notable exceptions. One such example is the early-type galaxy IC 2006 in the Fornax cluster. This galaxy has a remarkably axisymmetric HI ring extending from 0.5 - 6Re, with circular velocity nearly constant in this range of radii. In this galaxy, M / L rises strongly with radius, signifying a nearly axisymmetric dark halo [41].

Other ellipticals with settled HI disks are described in [42]. These galaxies probably acquired the HI gas by accretion. The dynamical interpretation requires understanding the shape of the potential. In several of these ellipticals, M / LB shows a substantial increase with radius, e.g., NGC 2974, NGC 3108, NGC 4278. In NGC 3108, again the rotation curve is flat, to 6Re, in agreement with the results from analysing stellar kinematics discussed above.