Quasars probing quasars. I. Optically thick absorbers near luminous quasars

With close pairs of quasars at different redshifts, a background quasar sight line can be used to study a foreground quasar's environment in absorption. We search 149 moderate-resolution background quasar spectra from Gemini, Keck, the MMT, and the SDSS to survey Lyman limit systems (LLSs) and damped Lyα systems (DLAs) in the vicinity of 1.8 < z < 4.0 luminous foreground quasars. A sample of 27 new quasar-absorber pairs is uncovered with column densities 10^17.2 cm^-2 < N_{H I} < 10 ^20.9 cm^-2 and transverse (proper) distances of 22 h ^-1 kpc < R < 1.7 h^-1 Mpc from the foreground quasars. If they emit isotropically, the implied ionizing photon fluxes are a factor of ∼5-8000 times larger than the ambient extragalactic UV background over this range of distances. The observed probability of intercepting an absorber is very high for small separations: six out of eight projected sight lines with transverse separations R < 150 h^-1 kpc have an absorber coincident with the foreground quasar, of which four have N_{H I} > 10¹⁹ cm^-2. The covering factor of N_{H I} > 10¹⁹ cm^-2 absorbers is thus ∼50% (4/8) on these small scales, whereas ≲2% would have been expected at random. There are many cosmological applications of these new sight lines: they provide laboratories for studying fluorescent Lyα recombination radiation from LLSs; they constrain the environments, emission geometry, and radiative histories of quasars; and they shed light on the physical nature of LLSs and DLAs.