2018ApJ...860...41L

We present the first results from our survey of intervening and proximate Lyman limit systems (LLSs) at z ∼ 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_⊥ <= 150 kpc and line-of-sight velocity separations <=11,000 km s^–1. We construct a stacked ultraviolet (rest-frame wavelengths 700-2000 Å) 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 ∼30% decrease in flux at λ = 800-900 Å compared to a stack of brighter quasars not in pairs at similar redshifts, and lower values of the mean free path of the H I ionizing radiation for pairs (λ_mfp⁹¹² = 140.7 ± 20.2 h₇₀^–1 Mpc) compared to single quasars (λ_mfp⁹¹² = 213.8 ± 28 h₇₀^–1 Mpc) at the average redshift z ≃ 2.44. From the modeling of LLS absorption in these pairs, we find a higher (∼20%) incidence of proximate LLSs with log N_HI >= 17.2 at δv < 5000 km s^–1 compared to single quasars (∼6%). These two rates are different at the 5σ 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 halos where these close pairs reside.