The spectral and environment properties of $z\sim2.0-2.5$ quasar pairs
  We present the first results from our survey of intervening and proximate
Lyman limit systems (LLSs) at $z$$\sim$2.0-2.5 using the Wide Field Camera 3
on-board the Hubble Space Telescope. The quasars in our sample are projected
pairs with proper transverse separations $R_\perp$$\leq$150 kpc and line of
sight velocity separations $\lesssim$11,000 km/s. We construct a stacked
ultraviolet (rest-frame wavelengths 700-2000\AA) spectrum of pairs corrected
for the intervening Lyman forest and Lyman continuum absorption. The observed
spectral composite presents a moderate flux excess for the most prominent broad
emission lines, a $\sim$30% decrease in flux at $\lambda$=800-900\AA\ compared
to a stack of brighter quasars not in pairs at similar redshifts, and lower
values of the mean free path of the HI ionizing radiation for pairs
($\lambda_{\rm mfp}^{912}=140.7\pm20.2~h_{70}^{-1}$Mpc) compared to single
quasars ($\lambda_{\rm mfp}^{912}=213.8\pm28~h_{70}^{-1}$Mpc) at the average
redshift $z\simeq2.44$. From the modelling of LLS absorption in these pairs, we
find a higher ($\sim$20%) incidence of proximate LLSs with $\log N_{\rm
HI}\geq17.2$ at $\delta v$$<$5,000 km/s compared to single quasars ($\sim$6%).
These two rates are different at the 5$\sigma$ level. Moreover, we find that
optically-thick absorbers are equally shared between foreground and background
quasars. Based on these pieces of evidence, we conclude that there is a
moderate excess of gas absorbing Lyman continuum photons in our
closely-projected quasar pairs compared to single quasars. We argue that this
gas arises mostly within large-scale structures or partially neutral regions
inside the dark matter haloes where these close pairs reside.
