QUASARS PROBING QUASARS. VI. EXCESS Hi ABSORPTION WITHIN ONE PROPER Mpc OF z ∼ 2 QUASARS

With close pairs of quasars at different redshifts, a background quasar sightline can be used to study a foreground quasar's environment in absorption. We use a sample of 650 projected quasar pairs to study the H I Ly alpha absorption transverse to luminous, z similar to 2 quasars at proper separations of 30 kpc < R-perpendicular to < 1 Mpc. In contrast to measurements along the line-of-sight, regions transverse to quasars exhibit enhanced H I Ly alpha absorption and a larger variance than the ambient intergalactic medium, with increasing absorption and variance toward smaller scales. Analysis of composite spectra reveals excess absorption characterized by a Ly alpha equivalent width profile W = 2.3 angstrom (R-perpendicular to/100 kpc)(-0.46). We also observe a high (similar or equal to 60%) covering factor of strong, optically thick H I absorbers (H I column N-H I > 10(17.3) cm(-2)) at separations R-perpendicular to < 200 kpc, which decreases to similar to 20% at R-perpendicular to similar or equal to 1Mpc, but still represents a significant excess over the cosmic average. This excess of optically thick absorption can be described by a quasar-absorber cross-correlation function xi(QA)(r) = (r/r(0))(gamma) with a large correlation length r(0) = 12.5(-1.4)(+2.7) h(-1) Mpc (comoving) and gamma = 1.68(-0.30)(+0.14). The H I absorption measured around quasars exceeds that of any previously studied population, consistent with quasars being hosted by massive dark matter halos M-halo approximate to 10(12.5) M-circle dot at z similar to 2.5. The environments of these massive halos are highly biased toward producing optically thick gas, and may even dominate the cosmic abundance of Lyman limit systems and hence the intergalactic opacity to ionizing photons at z similar to 2.5. The anisotropic absorption around quasars implies the transverse direction is much less likely to be illuminated by ionizing radiation than the line-of-sight.