ABSTRACT

With close pairs of quasars at different redshifts, a background quasar sightline can be used to study a foreground quasar in Ly-alpha absorption. This novel experiment allows us to probe the foreground quasar environment on scales as small as a galactic disk where the ionizing flux from the quasar could be as large as ~ 10,000 times the extragalactic UV background. I will discuss the manifold cosmological applications of these rare projected sightlines: they provide new laboratories for studying the faint fluorescent recombination radiation from the high redshift Universe, they constrain the environments, emission geometry, and radiative histories of quasars, and they shed light on the distribution and kinematics of the gas in high redshift proto-galaxies.