An Active Galaxy Cluster Merger at Cosmic Noon Revealed by JWST Weak Lensing and Multiwavelength Probes

March 2026 • 2026ApJ...999L...1S

Authors • Scofield, Zachary P. • Finner, Kyle • Joo, Hyungjin • Jee, M. James • Lee, Wonki • Cha, Sangjun • Kim, Jinhyub • Lin, Yu-heng • Chary, Ranga-Ram • Faisst, Andreas • Lee, Bomee

Abstract • The galaxy cluster XLSSC 122 is a rare system at z = 1.98, hosting surprisingly evolved member galaxies when the Universe was only one-third of its present age. Leveraging deep JWST/NIRCam imaging, we perform a weak-lensing analysis and reconstruct the cluster's mass distribution, finding a mass peak that coincides with both the X-ray peak and the position of the brightest cluster galaxy. Consistent with recent strong-lensing analyses, we obtain a mass estimate of 1.60 ± 0.30 (stat.) ± 0.26 (LSS) × 10¹⁴ M_⊙ and an implied concentration of c_200c = 6.3 ± 0.4, where the uncertainty represents the propagation of the mass error through the adopted concentration─mass relation and excludes intrinsic scatter. Placing our weak-lensing mass map in the context of Chandra X-ray data, MeerKAT radio imaging, Atacama Large Millimeter/submillimeter Array + Atacama Compact Array/Atacama Cosmology Telescope Sunyaev─Zel'dovich (SZ) mapping, and new JWST intracluster light measurements, we identify consistent NE─SW elongation across datasets and a pronounced offset along the same axis between the SZ and mass/X-ray peaks, pointing to significant merger activity. XLSSC 122 thus serves as a JWST pilot study for high-z lensing, demonstrating the telescope's unique ability to map cluster mass distributions at z ∼ 2. The concordance of the multiwavelength analysis here, together with the high concentration relative to ΛCDM expectations, motivates a uniform sample of analogous systems with joint lensing, X-ray, SZ, and radio data to probe cluster assembly at cosmic noon.

Links

• ELECTR https://doi.org/10.3847/2041-8213/ae447a
• PREPRINT http://arxiv.org/abs/2512.11022