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, $\rho_{\rm cl} \approx (1.6-4.7) r_{500}$)
of three massive ($M_{500}\sim3\times10^{14} M_{\odot}$) clusters of galaxies
at redshift $z_{\rm cl} \approx 0.46$, in the $Hubble Space Telescope$ Cosmic
Origins Spectrograph ($HST$/COS) spectra of 3 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 partial
Lyman limit absorber with $N(HI) > 10^{16.5} \ \rm cm^{-2}$ near the
photometric redshifts ($|\Delta z/(1+z)| \approx 0.03$) of the clusters. The
compound probability of 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
single cloud curve-of-growth (COG) analysis, suggest a non-thermal/turbulent
velocity of a few $\times10 \ \rm km s^{-1}$ in the absorbing gas. We emphasize
the need for uniform galaxy surveys around these fields and for more UV
observations of QSO-cluster pairs in general in order to improve the statistics
and gain further insights into the unexplored territory of the largest
collapsed cosmic structures.
