On the Generating Mechanism of Spiral Structure

Flat galaxies want to transfer their angular momentum outwards. Only in trailing spiral structures do the gravity torques carry angular momentum outwards. We show that the presence of waves lowers the angular momentum in the inner parts and increases it in the outer parts. Further there is absorption of angular momentum by stars that resonate with the wave at the outer Lindblad resonance, and at corotation. Emission of angular momentum occurs at the inner Lindblad resonance. The role of spiral structure is to carry angular momentum from the inner parts to the outer parts so that the waves may grow. We consider bars in galaxies to be standing waves which have grown enough to orient and trap the major axes of orbits with two lobes so that they lie along the bar.