Quasar absorption line systems are redshift-independent sensitive mass tracers .
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 cluster/group candidates from the Red-Sequence Cluster Survey .
In a common field of \approx 20 square degrees , we have found 442 quasar-cluster pairs for which the Mg ii \lambda \lambda 2796 , 2803 Å doublet might be detected at a transverse ( physical ) distance < 2 h ^ { -1 } _ { 71 } 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 \Delta z _ { cluster } = 6.3 and \Delta z _ { cluster } = 57.0 for the two samples , respectively .
We estimate the completeness level to be nearly 100 % for W detection thresholds of W _ { 0 } ^ { 2796 } > 0.05 and W _ { 0 } ^ { 2796 } > 1.0 Å in the two samples , respectively .

The results are : ( 1 ) the population of strong Mg ii systems ( W _ { 0 } ^ { 2796 } > 2.0 Å ) near cluster redshifts shows a significant ( > 3 \sigma ) overabundance ( up to a factor of 15 ) when compared with the ’ field ’ population ; ( 2 ) the overabundance is more evident at smaller distances ( d < 1 h ^ { -1 } _ { 71 } Mpc ) than larger distances ( d < 2 h ^ { -1 } _ { 71 } Mpc ) from the cluster center ; and , ( 3 ) the population of weak Mg ii systems ( W _ { 0 } ^ { 2796 } < 0.3 Å ) near cluster redshifts conform to the field statistics .
Unlike in the field , this dichotomy makes n ( W ) in clusters appear flat and well fitted by a power-law in the entire W -range .
We assess carefully all possible selection and systematic effects , and conclude that the signal is indeed due to the presence of clusters .
In particular , a sub-sample of the most massive clusters yields a stronger and still significant signal .
Since either the absorber number density or filling-factor/cross-section affects the absorber statistics , an interesting possibility is that we have detected the signature of truncated halos due to environmental effects .
Thus , we argue that the excess of strong systems is due to a population of absorbers in an overdense galaxy environment , and the lack of weak systems to a different population , that got destroyed in the cluster environment .

Finally , comparison with models of galaxy counts show that there is proportionally less cold gas in more massive clusters than in low-mass systems , and two orders of magnitude less Mg ii cross-section due to weak systems than due to stronger systems .