  The hosts of luminous z~2 quasars evolve into today's massive elliptical
galaxies. Current theories predict that the circum-galactic medium (CGM) of
these massive, dark-matter halos (M~10^12.5 Msun) should be dominated by a
T~10^7 K virialized plasma. We test this hypothesis with observations of 74
close-projected quasar pairs, using spectra of the background QSO to
characterize the CGM of the foreground one. Surprisingly, our measurements
reveal a cool (T~10^4 K), massive (M_CGM > 10^10 Msun), and metal-enriched (Z >
~0.1 Zsun) medium extending to at least the expected virial radius (r_vir = 160
kpc). The average equivalent widths of HI Lya (<W_lya> = 2.1 pm 0.15Ang for
impact parameters R<200 kpc) and CII 1334 (<W_1334> = 0.7 pm 0.1Ang) exceed the
corresponding CGM measurements of these transitions from all galaxy populations
studied previously. Furthermore, we conservatively estimate that the quasar CGM
has a 64% covering fraction of optically thick gas (N_HI>10^17.2) within r_vir;
this covering factor is twice that of the contemporaneous Lyman Break Galaxy
population. This unexpected reservoir of cool gas is rarely detected
"down-the-barrel" to quasars, and hence it is likely that our background
sightlines intercept gas which is shadowed from the quasar ionizing radiation
by the same obscuring medium often invoked in models of AGN unification.
Because the high-z halos inhabited by quasars predate modern groups and
clusters, these observations are also relevant to the formation and enrichment
history of the intragroup/intracluster medium.
