Introduction

We present a simple analytic description of the evolution of a captured galactic gas disc as it settles into a preferred orientation. These discs, which often display warps and twists (e.g. Cen A), are widely believed to arise from the tidal capture of gas from a nearby galaxy or the accretion of a gas rich companion. The newly formed disc will initially be unstable and evolve on both the precessional and viscous timescales. Differential precession causes a smooth continuous twist to develop. Cloud-cloud collisions within this twisted disc lead to the transport of angular momentum, causing changes in the orientations of cloud orbits and the inflow of material. Ultimately, the disc settles into a preferred orientation.

Steiman-Cameron & Durisen (1988) (hereafter SCD) derived three coupled differential equations governing the evolution of annular mass elements of a fluid disc including the effects of both orbit precession and viscosity. These equations, which describe the transport of angular momentum by Navier-Stokes stresses, describe the motion of orbit-averaged annular fluid elements. In general these equations must be solved numerically. However, analytic solutions are possible when: (1) disc precession is dominated by gravitational forces, with viscous forces being a minor perturbation. This condition is generally met. (2) The viscous timescale for inflow is much longer than the timescale for disc settling. This is true if condition four is met. (3) Settling takes place in an axisymmetric galaxy. However, if the initial disc inclination relative to a preferred orientation is small, then this condition is often approximately met even in triaxial potentials.