Quasars Probing Quasars. I. Optically Thick Absorbers near Luminous Quasars

With close pairs of quasars at different redshifts, a background quasarsightline 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 Ly-alphasystems (DLAs) in the vicinity of 1.8 < z < 4.0 luminous foreground quasars. Asample of 27 new quasar-absorber pairs is uncovered with column densities, 17.2< log (N_HI/cm^2) < 20.9, and transverse (proper) distances of 22 kpc/h < R <1.7 Mpc/h, from the foreground quasars. If they emit isotropically, the impliedionizing photon fluxes are a factor of ~ 5-8000 times larger than the ambientextragalactic UV background over this range of distances. The observedprobability of intercepting an absorber is very high for small separations: sixout of eight projected sightlines with transverse separations R < 150 kpc/hhave an absorber coincident with the foreground quasar, of which four have logN_HI > 10^19. The covering factor of log N_HI > 10^19 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 sightlines: they providelaboratories for studying fluorescent Ly-alpha recombination radiation fromLLSs, constrain the environments, emission geometry, and radiative histories ofquasars, and shed light on the physical nature of LLSs and DLAs.Comment: 27 pages (10 pages of figures), 5 tables, submitted to Ap