Abstract
N-body simulations show that bars form naturally in galactic discs. This can happen even if the discs are sub-maximum, due to the contribution of the halo resonant particles. The halo is not rigid, but responds to the bar and some of its mass is in resonance with the bar. The disc particles trapped around the closed periodic orbits of the x[1] family, or around the 3D orbits that bifurcate from it, emit angular momentum, which is absorbed either by particles at resonance in the outer disc or by resonant halo particles. Since the bar is a negative angular momentum perturbation, the more angular momentum it loses, the stronger it will become. Thus the strongest bars will be found in cases where the amount of angular momentum exchanged has been largest. I discuss the disc and halo properties that determine the amount of angular momentum exchanged.