AGN feedback appears necessary to explain not only the colors and luminosities of massive galaxies but also the X-ray luminosity-temperature relations observed among massive elliptical galaxies, galaxy groups, and clusters of galaxies. Perhaps counterintuitively, AGN feedback is also necessary to stabilize the “cool cores’’ commonly found in the centers of galaxy clusters. Studies of how AGN feedback works in clusters of galaxies may therefore shed light on how feedback works in lower-mass systems. We have made progress in understanding this intriguing connection, both in observation and theory. In massive galaxies and in clusters of galaxies, the thermodynamic state of the circumgalactic gas is closely linked with the presence of multiphase, potentially star-forming, gas in the central galaxy. I will discuss the observational evidence for this claim and I will describe a useful model for thinking about feedback in the context of condensation and precipitation of circumgalactic gas. I will also discuss implications for lower mass galaxies if feedback triggered by precipitation plays an important role in those galaxies as well.
