Theoretical Astrophysics, 130-33 California Institute of Technology, Pasadena, California 91125
Galaxies with elevated metabolic rates get energy from their gaseous food by extracting its nuclear energy (in stars), and its gravitational energy (via accretion onto massive black holes). There is strong evidence that interactions with other galaxies trigger star formation activity, and weaker evidence that it triggers black hole accretion (nuclear activity). We review the processes by which interactions can remove angular momentum from gas, particularly gravitational torques and the m=2, m=1, Jeans and fission instabilities that give rise to them. There is ample evidence, both theoretical and observational, that these can remove enough angular momentum to move much of a galaxy's gas from ~3kpc to ~300pc. This is still many decades from the ~10^{-5} pc scales of stars and black hole horizons. We discuss star formation, the interpretation of simulations, and cosmological implications. The evolution of binary supermassive black holes, and the problem of forming a dense (< 1 pc) nuclear star cluster are examined.
