Kinematic Mapping of Giant Arcs: A New Method to Locate Lensing Critical Curves

TL;DR

This paper introduces a novel kinematics-based method to precisely locate lensing critical curves in galaxy clusters, improving understanding of highly magnified sources and dark matter substructures.

Contribution

The authors develop and validate a new technique using integral field spectroscopy to locate critical curves with high accuracy, applicable to complex galaxy cluster lenses.

Findings

Applied method to Abell 370, constraining critical curve within 0.23" uncertainty.

Future JWST observations could reduce uncertainty to 0.08".

Method can detect small-scale dark matter substructures through critical curve wiggles.

Abstract

Proximity of lensing critical curves features highly magnified portions of lensed galaxies. Accurate knowledge of the location and shape of the critical curve will be useful for understanding the nature of highly magnified stellar sources near critical curves and for revealing sub-galactic dark matter structures within the lens. In galaxy-cluster lenses, however, prediction of critical curves can be uncertain due to complexity in global mass modeling. We explore and validate a kinematics-based method for locating the critical curve. This method leverages the continuous line-of-sight velocity profile of the lensed galaxy mapped through integral field spectroscopy of emission lines, and combines an agnostic local lens model and a disk rotation model. Applying our method to a highly magnified region of the Dragon Arc in the Abell 370 cluster lensing field using archival VLT/MUSE IFU mapping of the H$\beta$ line, we constrain the critical curve to an uncertainty band with a half-width of 0.23" ($1\sigma$). This result reveals locations of recently detected extremely magnified stars biased toward the negative-parity side of the critical curve, as predicted for intracluster microlensing. With future JWST/NIRSpec IFU mapping of the H$\alpha$ line at SNR $\simeq$ 10 (20), uncertainty could improve to 0.12" (0.08"). A measurement of this type with sufficiently small uncertainty may reveal small-scale wiggles in the shape of the critical curve, which can arise from the lensing perturbation of sub-galactic dark matter substructure. Our approach is generally applicable to caustic-crossing giant arcs and can be incorporated into global lens modeling.