  The universe's biggest galaxies have both vast atmospheres and supermassive
central black holes. This article reviews how those two components of a large
galaxy couple and regulate the galaxy's star formation rate. Models of
interactions between a supermassive black hole and the large-scale atmosphere
suggest that the energy released as cold gas clouds accrete onto the black hole
suspends the atmosphere in a state that is marginally stable to formation of
cold clouds. A growing body of observational evidence indicates that many
massive galaxies, ranging from the huge central galaxies of galaxy clusters
down to our own Milky Way, are close to that marginal state. The gas supply for
star formation within a galaxy in such a marginal state is closely tied to the
central velocity dispersion (sigma_v) of its stars. We therefore explore the
consequences of a model in which energy released during blackhole accretion
shuts down star formation when sigma_v exceeds a critical value determined by
the galaxy's supernova heating rate.
