JWST Cycle 3 SLICE Treasury Program

• JWST Cycle 3 SLICE Treasury Program is a comprehensive survey leveraging JWST’s high spatial resolution and infrared sensitivity to study cluster evolution and dark matter.
• It integrates dedicated strong lensing observations with pure-parallel slitless spectroscopy to map baryonic features and constrain dark matter distributions with high precision.
• The program synergizes multi-instrument data to explore high-redshift, emission-line galaxies, providing practical insights into cosmic structure formation.

The JWST Cycle 3 SLICE (“Strong LensIng and Cluster Evolution”) Treasury Program constitutes an extensive, multi-faceted survey designed to exploit JWST’s high spatial resolution and infrared sensitivity for transformative studies of cluster evolution, dark matter, and high-redshift galaxy populations. Integrated with the SAPPHIRES “Slitless Areal Pure-Parallel HIgh-redshift Emission” Survey, SLICE implements both dedicated strong lensing observations and groundbreaking pure-parallel slitless spectroscopy to advance two principal science goals: mapping the build-up of baryonic tracers in clusters and constraining dark matter structures through precise strong lens modeling, while simultaneously conducting an unbiased census of emission-line galaxies in the early universe (Cerny et al., 21 Mar 2025, Hsiao et al., 6 May 2025).

1. Scientific Rationale and Goals

SLICE pursues a dual legacy objective. The first is to chart the growth of stellar mass in brightest cluster galaxies (BCGs), intra-cluster light (ICL), and globular cluster populations to understand galaxy assembly and baryonic signatures of underlying dark matter. The second is precision mapping of dark matter distributions within massive galaxy clusters ($M_{500} \gtrsim 2 \times 10^{14} M_\odot$) via strong gravitational lensing (SL), targeting spatial precisions better than a few kpc to critically evaluate predictions of $\Lambda$CDM and alternate dark sector models (including self-interacting dark matter) over $0.2 < z < 1.9$ (Cerny et al., 21 Mar 2025).

The SAPPHIRES component, embedded within SLICE, aims to address JWST’s primary high-redshift goals: transparent identification of emission-line galaxies at $z \gtrsim 5$—especially those with extremely low metallicities ($12+\log({\rm O/H}) < 7.0$)—to probe the overall “metallicity floor,” search for Population III-like progenitors, and characterize early phases of cosmic chemical enrichment (Hsiao et al., 6 May 2025).

2. Sample Selection and Observational Strategy

Cluster fields for SLICE are mass-selected from a parent pool of 182 clusters in SZ (SPT, ACT) and X-ray (Planck, ROSAT) catalogs. The core observational sample consists of 100 cluster fields, with the pilot comprising 14 clusters sampling $0.2546 \leq z \leq 1.064$ and $2 \lesssim M_{500} / (10^{14} M_\odot) \lesssim 12$. Four clusters (MACS J0027.8+2616, MACS J1621.4+3810, PSZ1 G091.83+26.11, SPT-CL J0516-5755) are newly modeled for strong lensing (Cerny et al., 21 Mar 2025).

For each cluster, NIRCam is deployed with two extra-wide filters—F150W2 (short-wavelength, 1.0–2.0 μm) and F322W2 (long-wavelength, 2.4–5.0 μm)—with integrated shallow exposures (1836 s per filter). Dithering strategies (IntramoduleX3 for 1/f noise mitigation, 3-point small-grid for enhanced F150W2 PSF sampling) yield an approximate depth of S/N~3 at AB=29. Mosaics are astrometrically tied to Gaia and co-registered with archival HST imaging (ACS/WFC3: F435W through F160W). Multimodal spectroscopic coverage exploits VLT/MUSE, Magellan, and Chandra ACIS I/S X-ray mapping. (Cerny et al., 21 Mar 2025)

The SAPPHIRES survey operationalizes pure-parallel NIRCam Wide-Field Slitless Spectroscopy (WFSS) whenever NIRSpec is prime (e.g., during MOS observations of MACS J0416.1–2403), covering $\sim16$ arcmin$^2$ per pointing (two modules) in the F356W and F444W filters using both orthogonal grisms (Grism-R, Grism-C), optimizing for [O III] and H$\beta$ emission from $z \sim 5$–9 galaxies for a cumulative on-sky time of 557 hr (Hsiao et al., 6 May 2025).

3. Lens Modeling and Analysis Methodologies

Strong lens models are constructed using Lenstool—a parametric, MCMC-driven code employing pseudo-isothermal elliptical mass distributions (dPIE/PIEMD) or NFW profiles parametrized by centroid $(x, y)$, ellipticity $\epsilon = (a^2-b^2)/(a^2+b^2)$, position angle $\theta$, core and cut radii $(r_{\rm core}, r_{\rm cut})$, and central velocity dispersion $\sigma_0$. Galaxy-scale halos follow scaling relations with optimization for those near lensed arcs. The lens equation and convergence are, respectively,

$$\beta = \theta - \nabla \psi(\theta), \quad \kappa(\theta) = \Sigma(\theta)/\Sigma_{\rm crit},$$

where $\Sigma_{\rm crit} = (c^2/4\pi G)(D_s/(D_l D_{ls}))$, and projected mass within $r$ is

$$M(<r) = \Sigma_{\rm crit} 2\pi \int_0^r \kappa(\theta) \theta d\theta.$$

Modeling iterates between lens-plane fits and constraint updates from predicted counter-images (Cerny et al., 21 Mar 2025).

Selected clusters are also analyzed with the free-form /WSLAP+ hybrid method, decomposing mass into Gaussian components on an adaptive grid with parametric galaxy scaling, by solving $\Phi = \Gamma X$ for the lensing potential under positivity. Comparisons show $M(<200~{\rm kpc})$ agreement between Lenstool and WSLAP+ to within $\lesssim4\%$ in overlapping cases (Cerny et al., 21 Mar 2025).

In SAPPHIRES, emission-line galaxy extraction applies dedicated reduction pipelines (removal of 1/f striping and artifacts), aperture photometry (Photutils), and optimal spectral extraction (Horne). Redshift confirmation uses $\chi^2(z)$ fitting against template line sets, cross-validated with photometry (Hsiao et al., 6 May 2025).

4. Key Discoveries and Early Results

Strong Lensing (SLICE)

• New SL constraints more than doubled for most clusters due to JWST’s $\sim$0.03″ resolution; 209 previously-unknown multiple image systems were identified across 14 fields, with up to 19 in a single cluster (RCS0327).
• Reconstructed mass profiles yield enclosed masses in the range $1.1$–$2.8 \times 10^{14} M_\odot$ (within 200 kpc) and $3.4$–$7.7 \times 10^{14} M_\odot$ (500 kpc), with rms positional residuals of 0.15″–0.82″.
• Projected mass and X-ray emission peaks generally agree to within a few arcseconds, supporting the accuracy of mass recovery except in complex, merging systems.
• Lensing cross section: The critical area for $\mu \geq 3$ magnification increases $20$–$100\%$ between $z_s = 2$ and $z_s = 9$, highlighting utility for $z>6$ source studies.
• A multiply-imaged transient (likely SN) was discovered in SPT-CL J0516-5755, with predicted time delays of up to $\sim4000$ days between images—enabling direct time-delay cosmography (Cerny et al., 21 Mar 2025).

Pure-Parallel Spectroscopy (SAPPHIRES)

• Seven extremely metal-poor galaxy candidates ($12 + \log({\rm O/H}) < 7.0$) were identified in the early data, including two with $Z < 1\% \, Z_\odot$, directly violating both the local ($Z < 1\% \, Z_\odot$) and previously observed high-$z$ ($Z < 2\% \, Z_\odot$) “metallicity floor.”
• Candidates are faint ($\sim28$–$30$ AB mag), low-stellar-mass ($\log (M_*/M_\odot) = 6.8$–$7.8$), with blue UV slopes ($-2.6 \lesssim \beta \lesssim -2.0$), indicative of low dust content ($A_V \lesssim 0.2$ mag) and young ages ($5$–$25$ Myr).
• SED-inferred star formation rates (SFR) are $0.2$–$2~M_\odot \, {\rm yr}^{-1}$ with sSFR$\sim5$–$100~{\rm Gyr}^{-1}$, and are offset $+0.5$ dex above the main sequence at $z\sim5$–$7$. Where H$\alpha$ is detected, SFR is 2–3$\times$ higher than SED estimates, consistent with recent intense bursts (Hsiao et al., 6 May 2025).

5. Data Products and Accessibility

All lens models and their outputs—critical curve and caustic maps, convergence ($\kappa$), shear ($\gamma$), magnification ($\mu$) maps, FITS mass maps, constraint catalogs, and member-galaxy listings—are public via the Strong Lensing Cluster Atlas Data Base (https://data.lam.fr/sl-cluster-atlas/). SAPPHIRES will deliver flux-complete, grism-selected galaxy samples, with Science-Ready data products formatted in FITS and ASCII parameter catalogs. Planned future releases include full strong+weak lensing decompositions and expanded model sets (Cerny et al., 21 Mar 2025, Hsiao et al., 6 May 2025).

6. Broader Implications and Cross-Survey Synergies

SLICE’s enhanced lensing constraint density makes it possible to systematically recover magnification-corrected luminosity functions at $z>9$, resolve star-forming clumps and proto-globular clusters on $\lesssim 100$ pc scales, and conduct direct tests of dark matter through BCG “wobble”, substructure census, and self-interaction effects. The synergy with ALMA, VLT/MUSE, HST (CLASH, RELICS, CANUCS, HFF), and forthcoming surveys (DES, Euclid, Rubin LSST, Roman) will enable holistic cluster and lensing analyses spanning the full baryonic and dark matter components across cosmic time (Cerny et al., 21 Mar 2025).

SAPPHIRES, by leveraging pure-parallel NIRCam WFSS, addresses selection biases inherent to targeted spectroscopy and is uniquely positioned to build a homogeneous census of extremely metal-poor and potentially Population III analog galaxies in the general field, informing models of early feedback and enrichment (Hsiao et al., 6 May 2025).

7. Future Prospects and Legacy Value

SLICE’s pilot data both validate its mass-selected, evolutionary approach and establish methodological benchmarks for future cluster and lensing surveys. As the program approaches completion of the 100-cluster sample, combined analyses will improve constraints on cluster physics, baryonic tracers, dark matter, and provide statistical support for time-delay cosmography using multiply-imaged supernovae. In parallel, SAPPHIRES programs in subsequent JWST cycles are poised to refine strong-line abundance calibrations, increase EMPG statistics, and approach direct spectroscopic confirmation of Population III-hosting systems, thus broadly fulfilling JWST’s early-universe science mandate (Cerny et al., 21 Mar 2025, Hsiao et al., 6 May 2025).