SPIRAL MODES DRIVEN BY NARROW FEATURES IN ANGULAR-MOMENTUM DENSITY

We use computer simulations to demonstrate that a galaxy disc is destabilized by many types of narrow axisymmetric feature in the angular-momentum density distribution of particles. The vigorously growing disturbances, which can develop even in discs having considerable random motion, have a large-scale spiral form. The instabilities associated with a narrow groove are of particular interest, but narrow ridges and features having more complicated profiles are also destabilizing. We present an approximate local analysis which reveals the mechanism for the modes and predicts frequencies for both groove and ridge modes in rough agreement with those from the global simulations.