1. The Stability of Polar Orbits
Three models have been proposed to explain the apparent stability of polar features in polar-ring galaxies. Accretion, either from tidal capture of matter from a nearby system or the merger of a gas rich companion, is generally agreed to be the source of the ring material. This material is spread into an annular disk or ring over a few orbital periods. The galactic gravitational field, dissipation, and the self-gravity of the accreted material play a role in the subsequent evolution of the ring. The proposed models differ in their interpretation of the importance and details of each of these. The preferred orientation model (e.g., Steiman-Cameron and Durisen 1982, Steiman-Cameron 1990) proposes that the gravitational potentials of S0 galaxies are slightly triaxial in nature. This small departure from axisymmetry will lead a fraction of all dissipative disks accreted by S0s to settle into a stable polar orientation. In the ring self-gravity model, Sparke (1986, 1990) demonstrated that a sufficiently massive ring captured at a high initial inclination in an axisymmetric galaxy modifies the gravitational potential of the galaxy in such a manner that differential precession in the disk can be eliminated, and the system can be quasi-stable. The statistical selection model (Schweizer, Whitmore and Rubin 1983) argues that polar rings may not be equilibrium structures, but rather that their structure is an artifact of the initial capture orientation. The apparent ring stability then results from the long timescale for differential precession to destroy the near planar appearance of the rings which are captured at high inclination.