GALAXY CLUSTERS IN THE LINE OF SIGHT TO BACKGROUND QUASARS. III. MULTI-OBJECT SPECTROSCOPY∗

Abstract

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 L´opez et al. The targets were selected based on the presence of an intervening Mgii absorption system at a similar redshift to that of a galaxy cluster candidate lying at a projected distance <2 h−1 71 Mpc from the QSO sight line (a “photometric hit”). The absorption systems span rest-frame equivalent widths between 0.015 and 2.028 Å. Our aim was three-fold: (1) to identify the absorbing galaxies and determine their impact parameters, (2) to confirm the galaxy cluster candidates in the vicinity of each quasar sightline, and (3) to determine whether the absorbing galaxies reside in galaxy clusters. In this way, we are able to characterize the absorption systems associated with cluster members. Our main findings are as follows. (1) We identified 10 out of 24 absorbing galaxies with redshifts between 0.2509 zgal 1.0955, up to an impact parameter of 142 h−1 71 kpc and a maximum velocity difference of 280 km s−1. (2) We spectroscopically confirmed 20 out of 31 cluster/group candidates, with most of the confirmed clusters/groups at z < 0.7. This relatively low efficiency results from the fact that we centered our observations on the QSO location, and thus occasionally some of the cluster centers were outside the instrument field of view. (3) Following from the results above, we spectroscopically confirmed of 10 out of 14 photometric hits within ∼650 km s−1 from galaxy clusters/groups, in addition to two 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 Δz = 0.1. The general population of our confirmed absorbing galaxies have luminosities LB ∼ L∗B and mean rest-frame colors (Rc − z ) typical of Scd galaxies. From this sample, 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 L´opez et al.