Mature but Still Growing: JWST Detection of the Earliest Intracluster Light at z ∼ 2

Journal article

We present a JWST analysis of intracluster light (ICL) in XLSSC 122 at z = 1.98, currently the most distant known strong-lensing galaxy cluster with an evolved member population. Using deep JWST imaging complemented by Hubble Space Telescope data and careful control of systematics, we robustly detect diffuse emission extending to several hundred kiloparsecs from the brightest cluster galaxy (BCG) down to ∼29magarcsec−2 . Multicomponent point-spread-function-convolved Sérsic modeling separates the surface brightness profiles into three components: a BCG core, a BCG envelope, and an ICL component, with stable Sérsic indices across wavelengths. Nearly flat color profiles indicate minimal radial variation in the stellar populations of the BCG envelope and the ICL. The median ICL fraction measured across seven bands is ∼17%, demonstrating that the buildup of intracluster stars in massive halos was already well underway by z ∼ 2. The ICL fraction peaks near 5000 Å in the rest frame, which provides the first confirmation at z > 1 that this characteristic rise is a feature of dynamically active clusters. We also detect a southern excess of ICL relative to the best-fit Sérsic model and quantify it using wavelet-based modeling, providing additional support that this system is dynamically active. The BCG + ICL light distribution and strong-lensing mass map show strong morphological agreement within ∼100 kpc. These findings establish the ICL as an early forming and dynamically informative component of massive halos.

Authors: Joo, H; Jee, MJ; Finner, K; Scofield, ZP; Cha, S

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Astronomical Society
• Journal: The Astrophysical Journal Letters
• Volume: 1002
• Issue: 1
• Pages: L17
• Article number: L17
• Publication date: 2026-04-22
• Acceptance date: 2026-04-06
• DOI: 10.3847/2041-8213/ae5c9f
• EISSN: 2041-8213
• ISSN: 2041-8205
• Language: English
• Keywords: Dark matter; James Webb Space Telescope; Hubble Space Telescope; High-redshift galaxy clusters; Galaxy interactions; Galaxy clusters