Expanding our View of Galaxies with Observations of the Circumgalactic Medium

The current model of cosmological structure formation is very successful at reproducing the observed large scale distribution of matter in our Universe. However, the formation of galaxies within It is still poorly understood. For example, it is unclear how galaxies obtain gas from their intergalactic environment and convert it into stars. Furthermore, the processes of star formation & black hole growth release significant amounts of energy, which affect the gas distribution and motions inside galaxies, and which can drive gas out and back into their surroundings. The direct environment of galaxies - the `circumgalactic medium' (or CGM) - therefore provides us with a laboratory in which galaxy formation can be tested on scales that are not accessible with direct observations of galaxies. Only recently have the first observations of the CGM been reported. I will argue that the resulting `expanded' view of galaxies can help us address outstanding questions in galaxy formation, namely (1) how galaxies get their gas, and (2) how star formation affects gas in and around galaxies.