  Galaxy growth depends critically on the interplay between radiative cooling
of cosmic gas and the resulting energetic feedback that cooling triggers. This
interplay has proven exceedingly difficult to model, even with large
supercomputer simulations, because of its complexity. Nevertheless, real
galaxies are observed to obey simple scaling relations among their primary
observable characteristics. Here we show that a generic emergent property of
the interplay between cooling and feedback can explain the observed scaling
relationships between a galaxy's stellar mass, its total mass, and its chemical
enrichment level, as well as the relationship between the average orbital
velocity of its stars and the mass of its central black hole. These
relationships naturally result from any feedback mechanism that strongly heats
a galaxy's circumgalactic gas in response to precipitation of colder clouds out
of that gas, because feedback then suspends the gas in a marginally
precipitating state.
