The Evolution of Viscous Inclined Disks in Axisymmetric and Triaxial Galaxies

Abstract

We have used a set of equations developed by Pringle (1992) to follow the evolution of a viscous twisted disk in a galaxy-like potential which is stationary or tumbling relative to inertial space. In an axisymmetric potential, the disk settles to the equatorial plane at a rate determined largely by the coefficient $ν_2$ associated with shear perpendicular to the local disk plane. If the disk is initially close to the galaxy equator, the rate at which the inclination decays is well described by the analytic formula of Steiman-Cameron and Durisen (1988); in a highly inclined disk, `breaking waves' of curvature steepen as they propagate through the disk, rendering the numerical treatment untrustworthy. In a triaxial potential which is stationary in inertial space, settling is faster than in an oblate or prolate galaxy, since the disk twists simultaneously about two perpendicular axes. If the figure of the potential tumbles about one of its principal axes, the viscous disk can settle into a warped state in which gas at each radius follows a stable tilted orbit which precesses so as to remain stationary relative to the underlying galaxy.