  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 HI Lya absorption
transverse to luminous, z~2 quasars at proper separations of 30kpc < R < 1Mpc.
In contrast to measurements along the line-of-sight, regions transverse to
quasars exhibit enhanced HI Lya 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 Lya equivalent width profile W = 2.3A (R/100kpc)^-0.46. We
also observe a high (~60%) covering factor of strong, optically thick HI
absorbers (HI column log NHI > 17.3) at separations R<200kpc, which decreases
to ~20% at R~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+2.7-1.4 Mpc/h (comoving) and gamma =
1.68+0.14-0.30. The HI absorption measured around quasars exceeds that of any
previously studied population, consistent with quasars being hosted by massive
dark matter halos Mhalo~10^12.5 Msun at z~2.5. The environments of these
massive halos are highly biased towards 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~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, which we interpret in
terms of the same obscuration effects frequently invoked in unified models of
active galactic nuclei.
