Probing the Circumgalactic Medium

Over the next decade, astronomers are investing billions of dollars on
new instruments to study galaxy formation and evolution at large
cosmological lookback times. However, cosmological simulations of
galaxy formation cannot resolve the relevant physics required to
robustly predict the observables which have been the focus of studies
of high-redshift galaxies. I will argue that observations of the
diffuse gas in the outskirts of galaxies, the so called circumgalactic
medium, are essential for constraining the 'initial conditions' for
galaxy formation. Such observations provide a fruitful comparison to
theory, because hydrodynamics at moderate overdensities is much easier
to simulate than molecule or star-formation. I will present our recent
observational work on the circumgalactic medium, including two novel
experiments using close quasar pairs, which allow us to probe diffuse
gas around the lowest and highest mass galaxies.  Absorption line
modeling techniques will be reviewed, and I will summarize the
implications of these new measurements in the context of galaxy
formation and feedback models.