Caught in Swallowtails: Discovery of Two Swallowtail Image Formations in MS 0451.6-0305

TL;DR

This paper reports the first observation of multiple swallowtail gravitational lensing formations in a galaxy cluster, revealing the ability to magnify sub-parsec structures in distant galaxies across a wide redshift range.

Contribution

It presents the discovery of two swallowtail caustic configurations in gravitational lensing, demonstrating their potential to magnify tiny galaxy features at various cosmic epochs.

Findings

First observation of multiple swallowtail formations in a galaxy cluster

Magnifications up to 300 for small galaxy knots

Lensing-corrected sizes down to sub-parsec scales

Abstract

We report the discovery of two swallowtail image formations at $z=2.91$ and $z=6.70$ behind the galaxy cluster MS 0451.6-0305 in JWST-NIRCam imaging. We find that in both of the above lensed systems, the complex image morphology cannot be reproduced by simple fold/cusp caustics, and detailed lens modeling reveals higher-order swallowtail caustic configurations. In the $z=2.91$ lens system, a small part of the source galaxy (which itself is part of a galaxy group) containing atleast two compact knots sits inside the swallowtail caustic, producing a quadruply imaged arc. At two of the image positions of these knots, we infer point source magnifications of $\gtrsim 300$, implying lensing-corrected effective radii of $\lesssim 0.8-1.5$ pc. The $z=6.70$ system exhibits even more complex image formation. We therefore only use the most confidently identified counter-images of knots in this system as constraints in our lens modeling. The resulting model predicts magnifications $\sim20-200$ and lensing-corrected effective radii of $\lesssim 0.8-18.5$ pc for various knots. Together, these two systems represent the first example of observations of multiple swallowtail image formations in a single galaxy cluster and demonstrate the ability of swallowtail caustics to magnify individual substructures at sub-parsec scales, from intermediate redshifts to the first billion years of the Universe.