Gravitational Lensing of Variability-slected QSO by Galaxy Clusters

Abstract

Distribution and properties of QSOs behind galaxy clusters in the UKJ287 field were studied. QSOs were selected using variability criteria and are confined to $z \ge 0.4$ and $m_B \le 19.5$. No reddening or obscuration of background QSOs due to dust in clusters is detected. A statistically significant positive angular correlation between clusters ($\langle z\rangle \simeq 0.15$) and QSOs ($0.4 \le z \le 2.2$) on scales of several arcminutes is found. This number overdensity of QSOs behind clusters is ascribed to weak gravitational lensing by foreground galaxy clusters. The overdensity of QSOs increases as their limiting magnitude gets brighter in a way that would be expected from a simple lensing model derived using Boyle, Shanks \& Peterson (1988) QSO number counts and an average amplification of $A=2$ due to clusters. The implied amplification is substantially larger than would be expected from isothermal sphere clusters with a velocity dispersion $\sigma_v \sim 1000$ km/s.