Abstract
This lecture tackles the problem of feeding supermassive black holes. To explain the typical radiation of a quasar, at least 200 million solar masses of gas - a significant fraction of a galaxy's gas - would have to be swallowed during an activity phase of 100 million years. How can all the angular momentum of this matter be evacuated in such a short time? To do so, we need to exert torsional moments, on several scales. Bars in galaxies and gravitational instabilities, which form non-axisymmetries, can provide twisting torques. But we need to assume a whole cascade of such instabilities, bars or spirals, at various radius scales in the galaxy. Numerical simulations have shown that a bar in a galaxy exerts a negative torque, causing gas to flow towards the center, between the corotation and the internal Lindblad resonance. Positive torques cause gas to move in the opposite direction, out of this space. What's needed is a succession of such instabilities, either temporally or spatially. This is indeed what numerical simulations show : when enough gas has accumulated at the center, the kinematics of the galaxy are changed, and a secondary bar can be uncoupled inside the first bar, extending the fall of gas towards the center. These bars have a corotation that corresponds to the internal Lindblad resonance of the large bar. Observational diagnosis is easy, as star formation accelerates at the Lindblad resonance, forming a bright ring. Close to the black hole, the m = 2 instabilities of the bars are relayed by m = 1 instabilities (one arm, decentering), as the orbits become Keplerian, and the potential dominated by the central mass of the supermassive black hole.
