Ring formation in triaxial potentials

The formation of rings of gas in the outer regions of early-type galaxies is considered. It has previously been shown that the stable orbital planes lie perpendicular to the major and minor axes. Discs initially inclined in other directions will precess about one of the symmetry axes. The precession rate varies with radius and will thus cause the disc to warp. This induces a local viscosity which acts so as to oppose relative motion and causes the disc to settle into one of the preferred planes. Our simulations show a qualitatively different behaviour: viscosity is insufficient to prevent break-up of the disc and so gas clouds move to fill the available tube orbit. After a few precession times particles have differentially precessed through 180o at a given radius and collisions occur in the symmetry plane of the tube orbit forming a ring in situ. (author)