A Five-Stage Framework for Slitless Grism Spectra: Demonstrated on Euclid Q1 Strong-Lens Candidates and Ported to CEERS NIRCam

R. Tata

First listed 2026-04-03 | Last updated 2026-04-28

Abstract

Wide-field slitless grism spectroscopy is difficult in the co-spatial regime, where multiple sources share one cross-dispersion PSF element and catalogue-level decontamination (aXe/LINEAR/Grizli) does not apply -- the regime of strong lenses, binary AGN, and mergers in HST, JWST, Euclid, and Roman grism data. This paper presents a five-stage framework for per-source 1-D spectra and redshifts: distortion-calibrated extraction, Fourier/Sersic/NNLS deblending, robust-PCA cleaning, weighted friend-of-friend with Hungarian cross-orientation matching, and a Bayesian posterior layer. Of 579 published Euclid Q1 lens candidates, 473 fall in the NISP footprint. A 3" DESI DR1 Iron match gives 57 counterparts (51 ZWARN=0, 46 clean v6d anchors). Extending to 9200 A adds features below 12500 A in 312 source and 324 deflector spectra. Source/deflector features are detected in 467/471 of 473; 371 source redshifts are secure, 75 probable, 21 tentative. Adopted z_def is filled for 440/473 (55 DESI + 385 PHZ MODE_1). Catalogue: 342 complete, 29 source-only, 75 Silver, 21 Bronze, 6 none. Posteriors succeed for 398/473; DESI deflector coverage is 0.67+/-0.07 (46 anchors). Tiers platinum/gold/silver/bronze/none = 62/230/82/24/75. Median source 68% CI is dz=1.30; DESI |dz|<0.1 recovery is 35%. Source-side support is synthetic-only; the full deblender's real-data performance is unmeasured here. A ported v6c on 10 CEERS DR0.7 JWST/NIRCam targets improves median |dz|/(1+z) from 0.43 to 0.18, recovers 4/10 within 0.10, and contains truth in the 68% CI for 8/10 -- portability across instruments, not independent calibration. The release is a 473-row catalogue with deflector redshifts, posterior intervals, and quality tiers.

Short digest

Euclid/NISP SIR slitless spectra at Q1 lens positions yield spectroscopic redshifts for 461 of 579 candidates, including 419 secure source redshifts, 199 deflector redshifts, and 178 complete (zs, zd) pairs with 148 dual-grism confirmations. Reliability is anchored by cross-correlating the anti-parallel RGS000/RGS180 orientations and a tiered catalog (gold/silver/bronze), with medians zs≈1.59 and zd≈1.06. This constitutes the largest single-campaign spectroscopic lens characterization to date, achieved with no additional telescope time. Extrapolating the ~80% characterization rate to Euclid’s 14,500 deg² suggests ~100,000 galaxy-scale lenses with redshifts, enabling enclosed-mass work pending external validation.

Key figures to inspect

• Figure 1 — Redshift distributions: verify the median zs≈1.59 and zd≈1.06 and that all gold-complete points lie above the zs=zd line; the color-coding by line multiplicity shows why certain zs are flagged secure.
• Figure 2 — Tier and coverage breakdown: inspect counts for 148 gold-complete, 188 gold-source, 108 silver, 17 bronze, and the subset excluded as suspect high‑z; confirms how dual-orientation coverage boosts reliability.
• Figure 3 — High-SNR galleries in RGS000 vs RGS180: check that the same emission/absorption features align in both orientations (e.g., [O III], Hα for sources; Ca II triplet/Mg I b for deflectors), illustrating the dual-grism cross-confirmation.
• Figure 4 — Diversity across redshift: compare systems spanning nearby deflectors to high‑zs sources to see which rest-frame lines enter the 1.25–1.85 μm bandpass and how morphology (arcs/rings) tracks with spectral line sets.