STATISTICAL ASSOCIATION OF QSOS WITH FOREGROUND GALAXY CLUSTERS

We report a statistically significant overdensity of high redshift QSO's in the directions of foreground galaxy clusters. QSO's are taken from the Large Bright QSO Survey (LBQS) between 1.4 less than or equal to z less than or equal to 2.2 with a limiting magnitude of m(B)=18.5. Foreground clusters are regions within 6 Zwicky radii of small Zwicky clusters at a characteristic redshift of about z approximate to:0.2, covering about 40% of the total area surveyed (304 square degrees). The overdensity, defined as the ratio of the number density of QSO's in the directions of clusters (''association QSO's'') to that in the remainder of the fields (''background QSO's''), is equal to 1.7, and formally differs from unity at 4.7 sigma significance. The observed overdensity probably is not due to statistical variation in QSO density, intrinsic QSO-QSO and/or cluster-cluster autocorrelations, or patchy Galactic obscuration. We thus interpret this observation as being due to statistical gravitational lensing of background QSO's by galaxy clusters. However, this amplitude of overdensity behind clusters cannot be accounted for in any cluster lensing model if the background QSO number-magnitude counts are similar to the intrinsic (unlensed) counts, and is implausible in any conventional model of cosmic mass distribution.