Nested elliptical orbits and their resemblance to a rolling torus structure

Abstract

ABSTRA C T Wide dust lanes are believed to be a common feature of active galaxies, obscuring the bright central regions. Although these dust lane features appear to be long-lived, thick discs are widely believed to be unstable and tend to collapse into very thin discs, owing to high frictional dissipation caused by intersecting orbital paths. Individual gas and dust particles in the disc are believed to orbit the galactic centre, under purely gravitational forces, in precessing elliptical orbits which are inclined to the principal plane of the disc. If the eccentricity of the orbits is related in a simple way to the inclination of the orbital planes, then the orbital paths can be nested and need not intersect. In this special case, the motion of the disc will be ‘low-loss’ and almost free of friction. The envelope traced out by these nested orbits resembles a torus and there are severe restrictions on the permitted values of orbital elements. This torus executes a collective motion, it rotates toroidally (‘rolls round’) as it orbits the galactic centre. The rolling torus analogue of nested elliptical orbits is simple to model and predicts velocity dispersions all along the dust lane ‐ even in the line of sight to the nucleus. Evidence is presented for the dust lane of the radio galaxy Centaurus A being a torus which rolls as it orbits the centre. Non-circular velocities and ‘radial streaming motions’ are observed in the dust lane of Cen A at a range of wavelengths. The size of these anomalous velocities is in good numerical agreement with the predictions of this model. These velocities are difficult to explain in dust lane models based on circular orbits.