Gas Disks and Supermassive Black Holes in Nearby Radio Galaxies

numerical approaches they made the following conclusions: (i) Settling is a local
process, depending primarily on the local properties of the potential and the gas disk;
(ii) minimum settling times are on the order of two to three disk precession times
which is equivalent to ten to a few tens of rotation periods; (iii) that the actively
settling region is narrow, with a radial width of about 30% of its outer radius; and
(iv) radial inflow is enhanced during the active settling period.
This means that in our observations of gas disks in ellipticals we need not expect
all of the gas to be settled, even if some portions of it are, and it may not be easy to
identify which portions of the gas are settled and which are not without detailed 2D
kinematics. We might expect to see signs of inflow in the actively settling regions,
which means as the gas settles it may appear to be unstable in the gravitational
potential. This is possibly what we observe in NGC 2329.
Steiman-Cameron & Durisen (1988) describe disks which settle through differen-
tial precession and dissipation, picturing the disk as a cloud fluid, where cloud-cloud
interactions play an important role in the dissipation. In Chapter 5 we will see that
this model is compatible with some of the kinematic parameters of the gas. They
found that gravitational and Coriolis forces will twist disks that are not in the major
plane of the gravitation potential – compatible with observations discussed in the
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