DYNAMIC EVOLUTION OF HIGHLY INCLINED RINGS

We have used three computer programs (two smoothed-particle hydrodynamics codes and one Eulerian hydrodynamics code) to model the detailed dynamical evolution of gaseous rings inside external spheroidal potential wells. In this paper, we discuss rings highly inclined to the equatorial symmetry plane; this is relevant to polar rings observed around galaxies. Our primary objective is to clarify the possible evolution of the rings because, in the past, different numerical techniques have produced entirely different scenarios. We find that the evolution is not affected by the magnitude or type of viscosity but can be strongly dependent on the adopted initial conditions. Our main conclusion is that real polar rings are not expected to collapse into the nuclear region or to settle significantly toward the preferred equatorial orientation within one Hubble time unless the shapes of the surrounding halos are either extremely distorted or distinctly prolate.