RXJ0437+00: constraining dark matter with exotic gravitational lenses

We present the first strong-gravitational-lensing analysis of the galaxy cluster RX J0437.1+0043 (RXJ0437; z = 0.285). Newly obtained, deep MUSE observations, Keck/MOSFIRE near-infrared spectroscopy, and Hubble Space Telescope SNAPshot imaging reveal 13 multiply imaged background galaxies, three of them (at z = 1.98, 2.97, and 6.02, respectively) in hyperbolic umbilic (H-U) lensing configurations. The H-U images are located only 20-50 kpc from the cluster centre, i.e. at distances well inside the Einstein radius where images from other lens configurations are demagnified and often unobservable. Extremely rare (only one H-U lens was known previously) these systems are able to constrain the inner slope of the mass distribution - and unlike radial arcs, the presence of H-U configurations is not biased towards shallow cores. The galaxies lensed by RXJ0437 are magnified by factors ranging from 30 to 300 and (in the case of H-U systems) stretched nearly isotropically. Taking advantage of this extreme magnification, we demonstrate how the source galaxies in H-U systems can be used to probe for small-scale (similar to 10(9) M-circle dot) substructures, providing additional insight into the nature of dark matter.

Automated summary

This study presents the first strong gravitational lensing analysis of the galaxy cluster RXJ0437. By using deep MUSE observations, near-infrared spectroscopy, and Hubble Space Telescope imaging, the researchers discovered 13 multiply imaged background galaxies, including three in hyperbolic umbilic (H-U) lensing configurations. They were able to use these systems to determine the inner slope of the mass distribution and investigate the nature of dark matter.