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 1017.2 cm-2 < NH 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 NH I > 1019 cm-2. The covering factor of NH 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.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science