Nonaxisymmetric disk instabilities - A linear and nonlinear synthesis

Nonaxisymmetric instabilities in two-dimensional fluid disks orbiting black holes are studied in two dimensions using the results of linear and nonlinear perturbation studies. In general, two types of nonaxisymmetric behavior are found: 'planets' and spiral waves. The planets result from the principal mode and saturate at large perturbation amplitudes. Beyond the principal mode, the disk saturates with a lower-amplitude density enhancement near the pressure maximum with a trailing spiral wave. These two features represent the saturation of the nodeless inner surface wave and the outer sound wave, respectively. The changing angular momentum distribution drives accretion in disks whose inner edges are close to the black hole. The implications of the nonlinear results for the 'standard' theoretical picture of thick disks around black holes are considered.