|Title||Quasars probing quasars: thinking outside of the grid|
Over the next decade, astronomers will invest 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 provide a much more fruitful comparison to theory, because hydrodynamics at moderate overdensities is much easier to simulate than molecule or star-formation. I will introduce a novel technique whereby a foreground quasar (and massive galaxy) can be studied in absorption against a background quasar, resolving scales as small as 30kpc. This experiment reveals a rich absorption spectrum which contains a wealth of information about the physical conditions of diffuse gas around massive proto-galaxies. These types of measurements will allow us to determine the initial conditions for galaxy formation. Absorption line modeling techniques will be reviewed, and I will discuss the implications of these new observations for galaxy formation and feedback scenarios.