Quasars Probing Quasars IV: Joint Constraints on the Circumgalactic
  Medium from Absorption and Emission
  We have constructed a sample of 29 close projected quasar pairs where the
background quasar spectrum reveals absorption from optically thick HI gas
associated with the foreground quasar. These unique sightlines allow us to
study the quasar circumgalactic medium (CGM) in absorption and emission
simultaneously, because the background quasar pinpoints large concentrations of
gas where Ly-a emission, resulting from quasar-powered fluorescence, resonant
Ly-a scattering, and/or cooling radiation, is expected. A sensitive
slit-spectroscopic search (1-sigma limits of SB_Lya ~= 3e-18
erg/s/cm^2/arcsec^2) for diffuse Ly-a emission in the environments of the
foreground quasars is conducted. We fail to detect large-scale ~ 100 kpc Ly-a
emission, either at the location of the optically thick absorbers or in the
foreground quasar halos, in all cases except a single system. We interpret
these non-detections as evidence that the gas detected in absorption is
shadowed from the quasar UV radiation due to obscuration effects, which are
frequently invoked in unified models of AGN. Small-scale R_perp <~ 50 kpc
extended Ly-a nebulosities are detected in 34% of our sample, which are likely
the high-redshift analogs of the extended emission-line regions commonly
observed around low-redshift (z < 0.5) quasars. We also detect a compact high
rest-frame equivalent width (W_Lya > 50 A) Ly-alpha-emitter with luminosity
L_Lya =2.1+-0.32e41 erg/s at small impact parameter R_perp=134 kpc from one
foreground quasar, and argue that it is more likely to result from
quasar-powered fluorescence, than simply be a star-forming galaxy clustered
around the quasar. Our observations imply that much deeper integrations with
upcoming integral-field spectrometers such as MUSE and KCWI will be able to
routinely detect a diffuse Ly-a glow around bright quasars on scales R ~ 100
kpc and thus directly image the CGM. [abridged]
