Dynamical Evolution: Spirals and Bars
Non-axisymmetric modes like spirals and bars are the main driver of the evolution of disks, in transferring angular momentum, and allowing mass accretion. This evolution proceeds through self-regulation and feedback mechanisms, such as bar destruction or weakening by a central mass concentration, decoupling of a nuclear bar taking over the gas radial flows and mass accretion, etc.. These internal mechanisms can also be triggered by interaction with the environment. Recent problems are discussed, like the influence of counter-rotation in the m=1 and m=2 patterns development and on mass accretion by a central AGN. 1. Bar formation and destruction 1.1. Rapid evolution of disks Galaxy disks are far from stationary. They are unstable, with timescales that can be very short according to the radius. This ranges from Myr in the centers to Gyr in the outer parts, According to the environment, companions, mass accretion, etc.. normal modes can be excited. With the mass distribution of normal nearby galaxies, the most rapidly growing mode is the m=2 (spirals and bars). Trailing waves transfer angular momentum to the outer parts, and concentrate the mass. This mass concentration itself changes the development condition of the modes, and weakens them. Alternatively, the disk is heated by waves and the perturbation fades away, until the next excitation. The evolution of the disk is then controlled by recurrent waves, through regulation and feedback processes. One of these is related to gravitational instabilities, suppressed or favored by heating and gas cooling. When the disk is cold (Toomre Q parameter < 1), and therefore unstable to spiral and bar waves, it develops non-axisymmetric perturbations and gravity torques, that transfer the angular momentum outwards. The disk is progressively heated by the waves until the Q threshold is reached. A disk with only old stars cannot cool and will then remain stable, while a galaxy rich in gas will continue to be unstable by gas cooling. Young stars formed out of the gas reform a cool unstable stellar disk. The gravity torques that drive the gas inwards play the role of a viscosity; the galaxy disk can then be considered as an accretion disk. An exponential stellar disk can then be built, through star formation, if there is approximate 1