Galaxies can evolve from actively forming stars to quiescent states, with effectively no ongoing star formation, in many wild and wonderful ways. In dense environments such as galaxy clusters, quiescent galaxies dominate the population, although it is unclear which mechanisms are most important in halting, or ‘quenching' the star formation. In any environment, violent gaseous outflows can remove or cut off the fuel supply out of which to form stars. The vast halos of gas enveloping galaxies, the circumgalactic medium (CGM), must mediate these quenching processes. First, I will present results from both observations and simulations of massive galaxy groups and clusters that shed light on how the CGM, and likely galaxies themselves, are transformed in these punishing environments. Second, I will present observations from the Keck Cosmic Web Imager, wherein we have caught an outflow in the act, with detected emission extending far in the CGM.
Environment- and feedback-induced galaxy evolution: Perspectives from the circumgalactic medium