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

Journal article

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 c200c = 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.

Authors: Scofield, ZP; Finner, K; Joo, H; Jee, MJ; Lee, W

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Astronomical Society
• Journal: The Astrophysical Journal Letters
• Volume: 999
• Issue: 1
• Pages: L1
• Article number: L1
• Publication date: 2026-02-23
• Acceptance date: 2026-02-10
• DOI: 10.3847/2041-8213/ae447a
• EISSN: 2041-8213
• ISSN: 2041-8205
• Language: English
• Keywords: James Webb Space Telescope; High-redshift galaxy clusters; Weak gravitational lensing; Intracluster medium; Dark matter