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With close pairs of quasars at different redshifts, a background quasarsightline can be used to study a foreground quasar's environment in absorption.We use a sample of 650 projected quasar pairs to study the HI Lya absorptiontransverse to luminous, z~2 quasars at proper separations of 30kpc < R < 1Mpc.In contrast to measurements along the line-of-sight, regions transverse toquasars exhibit enhanced HI Lya absorption and a larger variance than theambient intergalactic medium, with increasing absorption and variance towardsmaller scales. Analysis of composite spectra reveals excess absorptioncharacterized by a Lya equivalent width profile W = 2.3A (R/100kpc)^-0.46. Wealso observe a high (~60%) covering factor of strong, optically thick HIabsorbers (HI column log NHI > 17.3) at separations R<200kpc, which decreasesto ~20% at R~1Mpc, but still represents a significant excess over the cosmicaverage. This excess of optically thick absorption can be described by aquasar-absorber cross-correlation function xi_QA(r) = (r/r_0)^gamma with alarge correlation length r_0 = 12.5+2.7-1.4 Mpc/h (comoving) and gamma =1.68+0.14-0.30. The HI absorption measured around quasars exceeds that of anypreviously studied population, consistent with quasars being hosted by massivedark matter halos Mhalo~10^12.5 Msun at z~2.5. The environments of thesemassive halos are highly biased towards producing optically thick gas, and mayeven dominate the cosmic abundance of Lyman limit systems and hence theintergalactic opacity to ionizing photons at z~2.5. The anisotropic absorptionaround quasars implies the transverse direction is much less likely to beilluminated by ionizing radiation than the line-of-sight, which we interpret interms of the same obscuration effects frequently invoked in unified models ofactive galactic nuclei.

Quasars Probing Quasars VI. Excess HI Absorption Within One Proper Mpc of z~2 Quasars