Discovery of a cool, metal-rich gas reservoir in the outskirts of z ~
  0.5 clusters
  We built the first-ever statistically significant sample of ~80,000
background quasar-foreground cluster pairs to study the cool, metal-rich gas in
the outskirts (>R500) of z~0.5 clusters with a median mass of ~10^14.2 M_sun.
The sample was obtained by cross-matching the SDSS cluster catalog of Wen & Han
(2015) and SDSS quasar catalog of Lyke et al. (2020). The median impact
parameter (rho_cl) of the clusters from the quasar sightlines is 2.4 Mpc
(median rho_cl/R500 = 3.6). A strong MgII, along with marginal FeII, absorption
is detected in the mean and median stacked spectra of the quasars with MgII
rest-frame equivalent widths (REWs) of 0.034\pm0.003A (11-sig) and
0.010\pm0.002A (5-sig), respectively. The MgII REW shows a declining trend with
increasing rho_cl and rho_cl/R500, but does not show any significant trend with
mass (M500) or redshift (zcl) within the small M500 and zcl ranges probed here.
The MgII absorption signal and the trends persist even if we exclude the
quasar-cluster pairs where the background quasars may be probing the
circum-galactic medium (CGM) of clusters' member galaxies. The MgII (and FeII)
absorption reported here is the first detection of its kind. It indicates the
presence of a cool, metal-rich gas reservoir surrounding galaxy clusters out to
several R500. We suggest that the metal-rich gas in the cluster outskirts arise
from stripped materials and that gas stripping becomes important out to large
clustocentric distances (> 3R500).
