MUSE/HST z measure of galaxies in RXJ0437+00

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 (10^9^M_{sun}_) substructures, providing additional insight into the nature of dark matter.