2017ApJ...846L...8M

We report on the detection of three strong H I absorbers originating in the outskirts (i.e., impact parameter, ρ_cl ~(1.6-4.7)r₅₀₀) of three massive (M₅₀₀∼3×10¹⁴M_☉) clusters of galaxies at redshift z_cl~0.46, in the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) spectra of three background UV-bright quasars. These clusters were discovered by the 2500 deg² South Pole Telescope Sunyaev-Zel'dovich (SZ) effect survey. All three COS spectra show a partial Lyman limit absorber with N(H i) > 10^16.5 cm^–2 near the photometric redshifts (| Δz/(1+z)| ~0.03) of the clusters. The compound probability of the random occurrence of all three absorbers is <0.02%, indicating that the absorbers are most likely related to the targeted clusters. We find that the outskirts of these SZ-selected clusters are remarkably rich in cool gas compared to existing observations of other clusters in the literature. The effective Doppler parameters of the Lyman series lines, obtained using a single-cloud curve-of-growth (COG) analysis, suggest a nonthermal/turbulent velocity of a fewx10 km s^–1 in the absorbing gas. We emphasize the need for uniform galaxy surveys around these fields and for more UV observations of quasar-cluster pairs in general in order to improve the statistics and gain further insights into the unexplored territory of the largest collapsed cosmic structures.