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 700–2000Å ) 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 =800–900Å 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 _ { mfp } ^ { 912 } = 140.7 \pm 20.2 ~ { } h _ { 70 } ^ { -1 } Mpc ) compared to single quasars ( \lambda _ { mfp } ^ { 912 } = 213.8 \pm 28 ~ { } h _ { 70 } ^ { -1 } Mpc ) at the average redshift z \simeq 2.44 .
From the modelling of LLS absorption in these pairs , we find a higher ( \sim 20 % ) incidence of proximate LLSs with \log N _ { HI } \geq 17.2 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 .