On Bars, Haloes, their Interaction and their Orbital Structure
  A live halo plays an active role in the formation and evolution of bars by participating in the angular momentum redistribution which drives the dynamical evolution. Angular momentum is emitted mainly by near-resonant material in the bar region and is absorbed mainly by near-resonant material in the halo and in the outer disc. This exchange determines the strength of the bar, the decrease of its pattern speed, as well as its morphology. Thus, contrary to previous beliefs, a halo can help the bar grow, so that bars growing in galaxies with responsive massive haloes can become stronger than bars growing in disc dominated galaxies. During the evolution the halo does not stay axisymmetric. It forms a bar which is shorter and fatter than the disc bar and stays so all through the simulation, although its length grows considerably with time. I discuss the orbital structure in the disc and the halo and compare it with periodic orbits in analytical barred galaxy potentials. A central mass concentration (e.g. a central black hole, or a central disc) weakens a bar and increases its pattern speed. The effect of the central mass concentration depends strongly on the model, being less strong in models with a massive concentrated halo and a strong bar. 
E. Athanassoula