Straight arcs in galaxy clusters

Abstract

There is sufficient evidence that giant luminous arcs observed in the cores of rich clusters are background galaxies that are distorted and magnified by the lensing action of the cluster. In at least four cases a subclass of these features lying close to the center of the cluster are observed to be straight arcs. A general lens model is presented in which such a linear morphology is produced when a cluster lensing a background galaxy has projected surface mass density close to, but higher than, the critical value for multiple imaging and the lensed galaxy lies on or very near to the lip caustic of the cluster lens. The expected rotational velocity profile of the background source galaxy is computed and its importance as a diagnostic probe is highlighted. Observational tests of the model and diagnostic value of such lens configurations are discussed.