The Incidence of Strong-Lensing Clusters in the Red-Sequence Cluster Survey

The incidence of giant arcs due to strong-lensing clusters of galaxies is known to be discrepant with current theoretical expectations. This result derives from a comparison of several cluster samples to predictions in the framework of the currently favored $\Lambda$CDM cosmology, and one possible explanation for the discrepancy is that this cosmological model is not correct. In this paper we discuss the incidence of giant arcs in the Red-Sequence Cluster Survey (RCS), which again shows significant disagreement with theoretical predictions. We briefly describe a total of eight strong lens systems, seven of which are discussed here for the first time. Based on the details of these systems, in particular on the ratio of single to multiple arc systems, we argue that it may be possible to explain this discrepancy in the currently favored cosmology, by modifying the details of the lenses themselves. Specifically, the high incidence of multiple arc systems and their overall high redshift suggests that a sub-population of the global cluster population is responsible for much of the observed lensing. The lack of lensing clusters at $z<0.64$ in the RCS indicates that a property associated with clusters at early times results in the boosted lensing cross sections; likely a combination of ellipticity and elongation along the line of sight, substructure, and changes in the cluster mass profiles is responsible. Cluster mass, which should evolve to globally higher values toward lower redshifts, is clearly not the most significant consideration for the formation of giant arcs.