The Evolution of Viscous Inclined Disks in Axisymmetric and Triaxial Galaxies

We have used a set of equations developed by Pringle (1992) to follow theevolution of a viscous twisted disk in a galaxy-like potential which isstationary or tumbling relative to inertial space. In an axisymmetricpotential, the disk settles to the equatorial plane at a rate determinedlargely by the coefficient $\nu_2$ associated with shear perpendicular to thelocal disk plane. If the disk is initially close to the galaxy equator, therate at which the inclination decays is well described by the analytic formulaof Steiman-Cameron and Durisen (1988); in a highly inclined disk, `breakingwaves' of curvature steepen as they propagate through the disk, rendering thenumerical treatment untrustworthy. In a triaxial potential which is stationaryin inertial space, settling is faster than in an oblate or prolate galaxy,since the disk twists simultaneously about two perpendicular axes. If thefigure of the potential tumbles about one of its principal axes, the viscousdisk can settle into a warped state in which gas at each radius follows astable tilted orbit which precesses so as to remain stationary relative to theunderlying galaxy.