Galaxy Clusters in the Line of Sight to Background Quasars - III
  Multi-Object Spectroscopy
  We present Gemini/GMOS-S multi-object spectroscopy of 31 galaxy cluster
candidates at redshifts between 0.2 and 1.0 and centered on QSO sight-lines
taken from Lopez et al. (2008). The targets were selected based on the presence
of a intervening MgII absorption system at a similar redshift to that of a
galaxy cluster candidate lying at a projected distance < 2 h^{-1}Mpc from the
QSO sight-line (a 'photometric-hit'). The absorption systems span rest-frame
equivalent widths between 0.015 and 2.028 angstroms. Our aim was 3-fold: 1)
identify the absorbing galaxies and determine their impact parameters, 2)
confirm the galaxy cluster candidates in the vicinity of each quasar sightline,
and 3) determine whether the absorbing galaxies reside in galaxy clusters. Our
main findings are: 1) the identification of 10 out of 24 absorbing galaxies
with redshifts up to 1.0955. 2) The spectroscopic confirmation of 20 out of 31
cluster/group candidates, with most of the confirmed clusters/groups at z <
0.7. 3) Following from the results above, the spectroscopic confirmation of 10
out of 14 photometric hits within ~ 650 km/s from galaxy clusters/groups, in
addition to 2 new ones related to galaxy group environments. These numbers
imply efficiencies of 71% in finding such systems with MOS spectroscopy. This
is a remarkable result since we defined a photometric hit as those
cluster-absorber pairs having a redshift difference dz = 0.1. Absorbing
cluster-galaxies hosting weak absorbers are consistent with lower star
formation activity than the rest, which produce strong absorption and agree
with typical MgII absorbing galaxies found in the literature. Our spectroscopic
confirmations lend support to the selection of photometric hits made in Lopez
et al. (2008).
