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

Abstract

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 HST data and careful control of systematics, we robustly detect diffuse emission extending to several hundred kpc from the brightest cluster galaxy (BCG) down to about 29 mag arcsec^-2. Multi component PSF convolved Sersic modeling separates the surface brightness profiles into three components: a BCG core, a BCG envelope, and an ICL component, with stable Sersic 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 about 17 percent, demonstrating that the buildup of intracluster stars in massive halos was already well underway by z about 2. The ICL fraction peaks near 5000 Angstrom in the rest frame, resembling the behavior observed in dynamically active clusters. We also detect a southern excess of ICL relative to the best fit Sersic 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 about 100 kpc. These findings establish the ICL as an early forming and dynamically informative component of massive halos.

Figures (7)

• Figure 1: Multiband SB Profiles and Model Decomposition. The top row displays HST observations (F814W, F105W, and F140W), while the bottom row presents JWST/NIRCam observations (F090W, F200W, F277W, and F356W). Black data points represent the observed median SB within elliptical bins, with error bars accounting for the quadrature sum of flux uncertainties and systematic sky error ($\sigma_{sky}$). Horizontal dot-dashed lines indicate the limiting SB ($\mu_{lim}$) for each respective band. The red solid line denotes the best-fit multi-component Sérsic model convolved with the telescope PSF. The model is decomposed into three distinct stellar components: the BCG core (blue dotted line), the BCG envelope (orange dot-dashed line), and the diffuse ICL (green dashed line). The last components (green dashed line) are utilized for the ICL analysis in this work. Lower subpanels show the residuals ($\Delta\mu$) between the observed data and the total model.
• Figure 2: Radial color profiles of the BCG+ICL derived from six adjacent filter pairs. Shaded regions represent the 68% confidence interval, incorporating both statistical flux uncertainties and systematic sky background errors ($\sigma_{sky}$). The inner gray region ($\lesssim 1$ kpc) indicates the area where differences in PSF widths between HST and JWST may induce artificial color gradients. Beyond this scale, the color profiles remain remarkably flat within the error margins, suggesting a spatially uniform stellar population across the BCG envelope and ICL. A notable exception is the F814W$-$F090W profile, which exhibits variation at $3 \lesssim \rm SMA \lesssim 10$ kpc, potentially indicating radial differences such as dust attenuation within the central galaxy (see text).
• Figure 3: ICL fraction ($f_{ICL}$) as a function of rest-frame wavelength. The red crosses represent the measurements for XLSSC 122 ($z=1.98$) across the seven observed bands. For comparison, pink (sky blue) points indicate $f_{ICL}$ in dynamically active (relaxed) clusters measured by Jimenez-Teja2018Jimenez-Teja2021deOliveria2022Jimenez-Teja2024deOliveria2025. The $f_{ICL}$ values for XLSSC 122 exhibit a characteristic local maximum near 4800 $\text{\AA}$, a pattern that closely resembles the trends found in dynamically active or merging systems.
• Figure 4: Localization of non-Sérsic diffuse emission in XLSSC 122. The left column presents JWST/NIRCam images (F200W, F277W, and F356W) following the subtraction of the best-fit 2D Sérsic ICL (diffuse) component, revealing a prominent residual signal to the south. The middle column displays the isolated model of this additional diffuse component as detected by the wavelet-based algorithm DAWIS. The right column shows the final residual maps after subtracting both the global Sérsic and local DAWIS models. This southern feature represents a significant excess of diffuse emission extending approximately 100 kpc from the BCG. Its spatial alignment with the overdensity of member galaxies and independent multi-wavelength asymmetries (X-ray, radio, and SZ) strongly suggests an origin in tidally stripped stars resulting from ongoing or recent dynamical interactions within the cluster.
• Figure 5: Comparison between the mass distribution and diffuse stellar light in XLSSC 122. Top left: The radial profile of the mass-to-light ratio ($\kappa/\mu$) is derived from the SL mass map and the F356W SB profile. The ratio follows a linear relation ($\kappa/\mu \propto r$) up to $r \sim100$ kpc, the regime where the SL model constraints are most robust. Right: A two-dimensional comparison between the SL mass distribution (red contours) and the smoothed stellar light distribution of the combined BCG + ICL + member galaxies (blue contours and gray scales) is presented. Bottom left: The orange curve illustrates the OC between the light and mass contours, while the green curve displays the MHD. Both metrics consistently indicate that the overall morphological similarity between mass and diffuse light components remains high up to $r\sim 180$ kpc. The slight decrease in similarly beyond $r\sim60$ kpc is attributed to the presence of the southern ICL excess and the lack of the SL constraints.