An atlas of predicted exotic gravitational lenses

Abstract

Wide-field optical imaging surveys will contain tens of thousands of new strong gravitational lenses. Some of these will have new and unusual image configurations, and so will enable new applications: for example, systems with high image multiplicity will allow more detailed study of galaxy and group mass distributions, while high magnification is needed to super-resolve the faintest objects in the high-redshift universe. Inspired by a set of six unusual lens systems [including five selected from the Sloan Lens ACS survey (SLACS) and Strong Lens Legacy Survey (SL2S), plus the cluster Abell 1703], we consider several types of multi-component, physically motivated lens potentials, and use the ray-tracing code glamroc to predict exotic image configurations. We also investigate the effects of galaxy source profile and size, and use realistic sources to predict observable magnifications and estimate very approximate relative cross-sections. We find that lens galaxies with misaligned discs and bulges produce swallowtail and butterfly catastrophes, observable as ‘broken’ Einstein rings. Binary or merging galaxies show elliptic umbilic catastrophes, leading to an unusual Y-shaped configuration of four merging images. While not the maximum magnification configuration possible, it offers the possibility of mapping the local small-scale mass distribution. We estimate the approximate abundance of each of these exotic galaxy-scale lenses to be ∼1 per all-sky survey. In higher mass systems, a wide range of caustic structures are expected, as already seen in many cluster lens systems. We interpret the central ring and its counter-image in Abell 1703 as a ‘hyperbolic umbilic’ configuration, with total magnification ∼100 (depending on source size). The abundance of such configurations is also estimated to be ∼1 per all-sky survey.