The formation and evolution of galaxies depend essentially on their dynamics, gravity and gas hydrodynamics. These phenomena control star formation and all associated processes, including feedback from supernova energy. Each galaxy grows in symbiosis with its supermassive black hole, lurking at its center, which becomes active through gas accretion on a cyclical basis, with typical periods of 40 million years.
The first galaxies to form are very rich in gas. By dissipation, they quickly form a rotating disk. Gradually, small galaxies interact and merge to form larger galaxies. Gas is consumed to form stars, and the merging of stellar systems is no longer dissipative. The disks are then destroyed, and angular momentum is cancelled and diluted, to form spheroidal systems with little or no rotation.
Disc galaxies with gas are mostly spiral. Spiral or bar waves are necessary for tangential forces and torques to exchange angular momentum, allowing the galaxy to accrete matter, concentrate and form stars. Bars and spirals are the real engine of evolution.
Large-scale galaxy surveys, notably the Sloan Digital Sky Survey (SDSS), have revealed a characteristic bimodality of galaxies: in a color-magnitude diagram, galaxies are distributed into a red sequence of more massive galaxies at the end of evolution, and a blue cloud of less massive galaxies still forming stars. The processes that explain the abrupt cessation of star formation in red galaxies are still poorly understood. The fraction of red galaxies increases with mass and environment. This could be the result of gas sweeping in due to dynamic pressure, tidal interactions, or gas stabilization due to bulge growth. Feedback phenomena from either active cores or supernovae appear to be reversible.