Discovery of an H I-rich Gas Reservoir in the Outskirts of SZ-effect-selected Clusters

Abstract

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)r ₅₀₀) of three massive ({M}₅₀₀∼ 3× {10}¹⁴ M _⊙) 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 (| {{Δ }}z/(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 few×10 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.