Galaxy Clusters in the Line of Sight to Background Quasars. I. Survey Design and Incidence of Mg II Absorbers at Cluster Redshifts

Quasar absorption line systems are redshift-independent sensitive mass trac-ers. Here we describe the first optical survey of absorption systems associated with galaxy clusters at z = 0.3 − 0.9. We have cross-correlated quasars from the third data release of the Sloan Digital Sky Survey with high-redshift clus-ter/group candidates from the Red-Sequence Cluster Survey. In a common field of ≈ 20 square degrees, we have found 442 quasar-cluster pairs for which the Mg II λλ2796, 2803 A ̊ doublet might be detected at a transverse (physical) dis-tance < 2 h−171 Mpc from the cluster centers. In addition, we have found 33 other pairs in the literature and we have discovered 7 new quasars with foreground clusters. To investigate the incidence (dN/dz) and equivalent-width distribution n(W) of Mg II systems at cluster redshifts, two statistical samples were drawn out of these pairs: one made of high-resolution spectroscopic quasar observations (46 pairs), and one made of quasars used in Mg II searches found in the literature (375 pairs). The total redshift path from an ad-hoc definition of ’cluster redshift path ’ is ∆zcluster = 6.3 and ∆zcluster = 57.0 for the two samples, respectively. W