Cooperation of orbital streams in disc galaxies

An orbit is said to cooperate with a potential well if it tends to deepen it by aligning with it. We derive analytical formulae that give the response of resonant orbits to non-axisymmetric perturbations. This makes it possible to quantify the level of cooperation in a given galaxy model. The qualitative response of orbits can be characterized by the sign of what we call the cooperation parameter. We discuss the relevance of our results for …