Outer parts of accreting discs around supermassive black holes in galaxy nuclei and quasars

If the mass of a solitary black hole and the accretion rate are sufficiently great the outer parts of the accreting disc become ''selfgravitating'' and may desintegrate into fragments. This fragmentation forms around the black hole a ring consisting of stars and gas. The ring continues to exist even after the accretion has ceased. The orbits of stars in the ring are approximately of Kepler type and their velocity dispersion is small. The mass of the ring can reach some per cent of the mass of the black hole