SLICE: JWST Insights on Strong Lensing and Cluster Evolution

• SLICE is a JWST initiative that maps galaxy clusters using strong gravitational lensing to probe dark matter, baryons, and cosmic evolution.
• It combines deep NIRCam imaging, ground-based spectroscopy, and X-ray observations to achieve sub-arcsecond precision in mass reconstructions and identify multiple lensed systems.
• The program advances our understanding of cluster assembly, subhalo demographics, and lensed transients, providing key tests for dark matter models and cosmological parameters.

The Strong LensIng and Cluster Evolution (SLICE) program is a major James Webb Space Telescope (JWST) initiative targeting massive galaxy clusters to map their mass distributions through strong gravitational lensing. SLICE aims to resolve the interplay between dark matter (DM), baryons, and cluster evolution across cosmic time, capitalizing on JWST’s near-infrared sensitivity, angular resolution, and wide field of view. By integrating JWST/NIRCam imaging with ground-based spectroscopy and complementary X-ray observations, SLICE delivers high-precision lensing mass reconstructions of cluster cores, probes subhalo structure, measures the evolution of cluster mass profiles and concentrations, and provides critical magnification maps for the paper of high-redshift galaxies, transients, and time-delay cosmography.

1. Scientific Objectives and Survey Design

SLICE is designed to:

• Track the co-evolution of cluster DM halos and stellar constituents, including brightest cluster galaxies (BCGs), intracluster light (ICL), globular cluster populations, and tidal features over $0.2 < z < 1.9$.
• Map the inner DM distribution in clusters to $<5\ \mathrm{kpc}$ resolution, enabling stringent tests of cold (ΛCDM) and alternative dark matter models.
• Characterize mass substructure at $L \lesssim 0.01\,L_*$ and explore the galaxy-scale subhalo function.
• Exploit clusters as “cosmic telescopes” to magnify faint background galaxies at $z \sim 2–10$ and detect strongly lensed transients, with a focus on multiply imaged supernovae (SN) and active galactic nuclei (AGN).
• Measure the assembly and concentration evolution of massive clusters, tying strong-lensing constraints to thermodynamics inferred from X-ray data and stellar kinematics.

The target pool comprises 182 massive clusters selected by Sunyaev-Zel’dovich (SZ) and X-ray survey proxies, of which over 100 have been observed to date. SLICE provides a homogeneous mass-selected sample to enable statistical studies of cluster growth, merger history, and the high-mass end of cosmic structure at fixed epochs (Cerny et al., 21 Mar 2025).

2. Data Acquisition and Reduction

SLICE relies on deep JWST/NIRCam imaging, supplemented by archival HST, VLT/MUSE, and ground-based optical surveys as well as Chandra and eROSITA X-ray data. Key aspects of the observational campaign include:

• NIRCam Imaging: The canonical SLICE strategy deploys wide filters (F150W2, F322W2) for sensitivity to $1$–$4\ \mu$m. Deeper programs, such as for MACS J0138.0$-$2155, utilize six wide bands (F115W, F150W, F200W, F277W, F356W, F444W) with total integrations up to $20\,\mathrm{ks}$ per cluster. Reductions use the JWST pipeline, custom 1/f noise mitigation, astrometric alignment to the Gaia DR3 frame, and drizzle-combination to $0.04''\,\mathrm{pix}^{-1}$ mosaics (Acebron et al., 12 Mar 2025).
• Spectroscopy: VLT/MUSE (∼4 hr per field) provides spectroscopic redshifts for both cluster members and multiple-image systems ($\Delta z \lesssim 10^{-4}$ accuracy).
• Complementary Datasets: HST ACS/WFC3 delivers high-resolution optical imaging; Chandra and eROSITA provide X-ray mapping of the intracluster medium (ICM), enabling hydrostatic mass profile derivation out to $R_{500}$.
• Photometric Redshifts: Where spectroscopic redshifts are unavailable, photometric redshifts are computed using multi-filter SED fitting with tools such as Prospector or Bagpipes and are constrained through lens-model self-consistency.

This multimodal data set underpins the identification of strongly lensed systems, assignment of redshift priors, and validation of mass model parameters.

3. Lensing Mass Modeling Methodologies

SLICE employs both parametric (e.g., dual pseudioisothermal elliptical [dPIE], Navarro–Frenk–White [NFW]) and free-form (Maximum-entropy ReconStruction, “MARS”) techniques to reconstruct cluster mass distributions. Methodological highlights include:

• Lens Equation: The single-plane approximation is adopted,

$$\boldsymbol{\beta} = \boldsymbol{\theta} - \boldsymbol{\alpha}(\boldsymbol{\theta}),$$

mapping observed image positions $\boldsymbol{\theta}$ to source-plane coordinates $\boldsymbol{\beta}$ via the total deflection field $\boldsymbol{\alpha}$.

• Mass Components: Cluster-scale halos are typically described as non-truncated dPIE or NFW profiles; galaxy-scale subhalos are singular circular dPIEs, scaled in mass by a Faber–Jackson–type relation,

$$\sigma_0 = \sigma_0^{\rm ref} \left( \frac{L}{L^{\rm ref}} \right)^{\alpha}$$

with $\alpha$ calibrated from cluster galaxy kinematics and typical $$\sigma_0^{\rm ref}\sim 330\,\mathrm{km}\,\mathrm{s}^{-1}$$ (Acebron et al., 12 Mar 2025).

• Strong Lensing Constraints: The models incorporate multiply imaged background galaxies (spectroscopic “gold” and high-confidence “silver” systems), as well as resolved substructures (“knots,” H II regions) within arcs. JWST’s spatial resolution increases the number of constraints by factors of 2–3 compared to legacy HST data (Cerny et al., 21 Mar 2025).
• Modelling Codes: Lenstool is used for parametric, Markov chain Monte Carlo (MCMC) minimization in the image plane; WSLAP+ extends hybrid or free-form approaches; MARS utilizes a free-form, pixelated grid regularized by cross-entropy and solved with Adam optimization, enabling reconstructions with $>10^5$ free parameters (Cha et al., 2023).
• Mass Model Uncertainties: Systematic and statistical uncertainties are quantified by building families of models with different cluster mass parameterizations, directly propagating measurement and modeling errors into positional and magnification predictions.

This flexible lens modeling framework is foundational to SLICE’s science, underpinning all cluster mass, substructure, and magnification inferences.

4. Principal Results: Cluster Mass Profiles, Substructure, and Lensed Transients

SLICE delivers significant advances in cluster mass mapping and transient science:

• Image-plane Precision: JWST-based models achieve image-plane rms residuals as low as $0.05''$ for SNe images and $0.36''$ cluster-wide (MACS J0138), surpassing previous accuracy by factors of 2–5 (Acebron et al., 12 Mar 2025). For Abell 2744, joint WL+SL maps reach $0.11''$ rms (Cha et al., 2023).
• Mass Measurements: Robust enclosed projected masses are measured, e.g.,

$$M(<60\,\mathrm{kpc}) = (2.89^{+0.04}_{-0.03}) \times 10^{13} M_\odot\ \text{(MACS J0138)},$$

$$M(<200\,\mathrm{kpc}) = (1.9 \pm 0.3) \times 10^{14}\,M_\odot\ \text{(SPT-CL J0546–5345)}.$$

Uncertainties are routinely $<5\%$ in the core regions—a regime essential for discriminating between dark matter models (Allingham et al., 11 Jul 2025, Acebron et al., 12 Mar 2025).

• Subhalo and Merger Structure: Kinematic anchoring of subhalo scaling relations tightens constraints on galaxy-scale halo abundance down to velocity dispersions $\sigma_0 \lesssim 200$ km s$^{-1}$. In clusters such as PSZ2 G118.46+39.32, the bimodal mass configuration (subcluster mass ratio 0.83) is directly revealed, and alignment with X-ray peaks indicates pre-merger geometry (Smith et al., 11 Nov 2025).
• Magnification Forecasts and Transient Science: Precision magnification predictions are vital for correction of observed SN and background galaxy brightnesses. For MACS J0138, SN image magnifications are systematically 3–5$\times$ higher than prior estimates, driven by enhanced subhalo calibration (Acebron et al., 12 Mar 2025). JWST’s resolution enables robust forecasts of SN time delays at $<0.05''$ accuracy, underpinning time-delay cosmography (H$_0$ constraints) and tests for flux ratio anomalies. A candidate transient in SPT-CL J0516–5755 (z$_{\rm host} \approx 2.5$) demonstrates SLICE’s capacity for discovering and predicting reappearance of lensed transients (Cerny et al., 21 Mar 2025).

5. Integration with X-ray and Weak Lensing: Total Cluster Potential and Cosmological Tests

SLICE leverages multiwavelength data to cross-calibrate lensing and non-lensing mass tracers:

• Hydrostatic X-ray Mass: Chandra and eROSITA ICM observations yield hydrostatic mass profiles using density and temperature modeling. For SMACS J0723.3–7327, the projected hydrostatic and lensing masses within the Einstein radius (128 kpc) agree at the $1\sigma$ level:

$$M_{\rm proj,X-ray}(<128\,\mathrm{kpc}) = (8.0 \pm 0.7) \times 10^{13} M_\odot,\quad\text{cf.}\quad (7.6\text{--}8.6) \times 10^{13} M_\odot\ \text{from lensing} [2210.00633].$$

• Joint Weak+Strong Lensing: In Abell 2744, the combined MARS WL+SL map resolves substructure at $\sim$10 kpc, revealing merger bridges and providing a unique handle on the baryon–DM spatial relation. Five principal mass peaks coincide within $2''$ of the brightest cluster galaxies despite the model being blind to light (Cha et al., 2023).
• Gravity and Cluster Physics: The measured radial acceleration relation in SMACS J0723 lies well above the McGaugh et al. (2016) spiral galaxy curve, indicating the RAR is not universal and disfavors MOND-like modifications with a single acceleration scale (Liu et al., 2022). The close X-ray–lensing agreement also demonstrates that, in relaxed clusters, both probe a consistent DM-dominated mass distribution.

6. Implications for Cluster and Galaxy Evolution, and High-Redshift Universe

SLICE’s high-precision lens models impact several major research frontiers:

• Cluster Assembly: The ability to map both the global and core mass distributions with few percent accuracy informs models of cluster concentration evolution, mass accretion, and the timeline of subcluster mergers. For z > 1 clusters such as SPT-CL J0546–5345, SLICE reveals that central masses are comparable to lower-redshift Frontier Fields clusters, confirming early assembly and persistent high concentrations (Allingham et al., 11 Jul 2025).
• Subhalo Mass Function: Subhalo mass spectra, calibrated with kinematic scaling, enable quantitative tests of $\Lambda$CDM predictions for the abundance and distribution of galaxy-scale DM substructure, including the search for dark subhalos via flux ratio anomalies.
• Magnified High-$z$ Galaxy Science: Accurate magnification maps extend the “lensing strength” for background sources. The area with $\mu>3$ at $z_s=9$ is up to twice that at $z_s=2$ (e.g., in RCS2 032727), enhancing the ability to detect and paper faint galaxies at reionization and cosmic noon.
• Lensed Transients and Cosmology: The combination of improved lens model accuracy and predicted time delays for multiply imaged SNe and AGN supports precision measurements of the Hubble constant ($H_0$), SN luminosity functions, and constraints on dark-energy parameters.

7. Comparisons, Limitations, and Future Directions

Compared with pre-JWST studies, SLICE’s uniform NIRCam imaging and improved spectroscopic completeness double or triple the number of identified lensing constraints per cluster, halve the image-plane rms, and reveal new counter-images and internal arc substructure (Cerny et al., 21 Mar 2025). Four of the initial 14 SLICE clusters had no prior models; the rest show clear performance improvements over HST-based mass maps.

Limitations flow from the absence of system-wide spectroscopic redshifts and potential systematics in mass modeling, especially in dynamically active or merging systems. Comprehensive follow-up with ground-based and JWST NIRSpec spectroscopy, as well as additional X-ray and weak-lensing mapping, are identified as urgent priorities for full exploitation of the SLICE sample (Smith et al., 11 Nov 2025).

A likely direction is the scaling up of free-form lensing reconstructions (e.g., MARS) across the SLICE cluster population, enabling population-level studies of assembly, DM physics, and the evolution of cluster cores from $z \sim 1.9$ to $z\sim0.2$. Combining robust lensing results with multi-phase ICM diagnostics and stellar dynamical modeling will produce the most comprehensive cluster evolution dataset for the JWST era.