JWST Discovery of Strong Lensing from a Galaxy Cluster at Cosmic Noon: Giant Arcs and a Highly Concentrated Core of XLSSC 122

Abstract Our observations with the James Webb Space Telescope have made the remarkable discovery of strong gravitational lensing arcs from XLSSC 122 ( z = 1.98)—setting the record for the most distant galaxy cluster that exhibits strong lensing. The discovery of giant arcs enables a strong-lensing analysis and a measurement of the concentration of the dark matter halo. We perform a strong-lensing analysis of the cluster and measure the radial projected mass density profile. Our measurements reveal an exceptionally high concentration in the core of XLSSC 122. A Navarro–Frenk–White profile fit to the inner 100 kpc estimates the concentration to be 6.3 ± 0.5. The high concentration of XLSSC 122 contributes to the emerging picture that massive structure formation in the early Universe may proceed more rapidly than standard models suggest. We estimate the mass within 100 kpc to be M ( R < 100 kpc) = 6.5 ± 0.7 × 10 13 M ⊙ and find M 200 c = 2.6 ± 1.1 × 10 14 M ⊙ .