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

We report on the detection of three strong HI absorbers originating in the outskirts (i.e., impact parameter, rho(cl) approximate to (1.6-4.7)r(500)) of three massive (M-500 similar to 3 x 10(14) M-circle dot) clusters of galaxies at redshift z(cl) approximate to 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 deg(2) 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 (vertical bar Delta z(1 + z)vertical bar approximate to 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 x 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.