From Grism to IFU: Revising the Redshift and Nature of the Massive Dusty Galaxy S1 with JWST and ALMA

Mengyuan Xiao, Longji Bing, Gabriel Brammer, Pascal A. Oesch, David Elbaz, Rui Marques-Chaves, Miroslava Dessauges-Zavadsky, Benjamin Magnelli, Rychard Bouwens, Emanuele Daddi, Maximilien Franco, Qiusheng Gu, Thomas Herard-Demanche, Garth Illingworth, Ivo Labbe, Danilo Marchesini, Jorryt Matthee, Romain A. Meyer, Rohan P. Naidu, Irene Shivaei, Pieter van Dokkum, Andrea Weibel, Christina C. Williams, Stijn Wuyts

First listed 2026-02-03 | Last updated 2026-02-03

Abstract

We report a revised spectroscopic redshift for the dusty massive galaxy S1, previously inferred with an exceptionally high baryon-to-star conversion efficiency from NIRCam slitless grism data at $z_{\rm grism}=5.58$. Our new JWST/NIRSpec IFU observations reveal multiple rest-frame optical and NIR emission lines, yielding a secure spectroscopic redshift of $z_{\rm spec}=3.2439\pm0.0002$. We show that the earlier grism-based redshift resulted from contamination by a nearby galaxy whose dispersed spectral trace overlaps with S1, illustrating a known challenge of slitless spectroscopy when only a single dispersion angle and single emission feature are available. In addition, we present new ALMA 1 mm observations, which robustly detect dust emission ($S_{\rm 1mm}=0.99\pm0.03$ mJy) and show a dust half-light radius ($R_{\rm e,1mm}=0.73\pm0.10$ kpc) slightly smaller than the stellar size ($R_{\rm e, F444W} = 0.97\pm0.01$ kpc). Using the revised redshift and compiled multi-wavelength photometry, we update the UV-to-FIR SED and find that S1 is less extreme than previously inferred, yet remains a very massive (log$M_{\star}/M_{\odot}\sim10.6$), heavily obscured star-forming galaxy. The updated SED modeling reveals S1 to be a very dust- and gas-rich system with a moderate star formation rate and a long gas depletion time ($τ_{\rm dep} \sim 1.4$ Gyr), deviating from SMGs and OFGs, but more closely resembling typical massive main-sequence galaxies. We note that, although this revision reduces the number of ultra-massive galaxies reported in Xiao et al. 2024, it does not alter the main conclusions of that work. Overall, our study clarifies the nature of S1 and underscores the importance of multi-line spectroscopic confirmation, slitless observations at multiple position angles, and IFU data for robust redshift and physical characterization of rare massive galaxies in the early Universe.

Short digest

NIRSpec IFU spectroscopy revises the dusty GOODS source S1 from a grism-based z=5.58 to a secure z_spec=3.2439±0.0002 via multiple rest-optical/NIR lines ([S III], He I 1.083 μm, Paγ). The team demonstrates that the original single-line slitless identification was contaminated by a neighboring galaxy whose dispersed Brγ trace overlapped S1, highlighting the risk of single-PA grism redshifts. New ALMA 1 mm imaging detects S_1mm=0.99±0.03 mJy and finds compact dust (Re_1mm=0.73±0.10 kpc) slightly smaller than the F444W stellar size (Re=0.97±0.01 kpc). With the revised redshift, S1 is a very massive (log M*≈10.6), heavily obscured, dust- and gas-rich main-sequence galaxy with a moderate SFR and a long gas depletion time (τ_dep≈1.4 Gyr).

Key figures to inspect

• Figure 1: Use the IFU 1D spectrum and model overlay to see the multi-line redshift solution at z=3.2439, confirming [S III], He I 1.083 μm, and Paγ in the G395H window and the continuum level that anchors the fit.
• Figure 2: Inspect zoomed line profiles to check centroids and S/N of [S III], He I 1.083 μm, and Paγ that set z_spec, and verify mutual consistency among lines without relying on a single feature.
• Figure 3: Compare the ALMA 1 mm map and contours with the JWST RGB to gauge dust–starlight co-spatiality; note the compact dust size (Re_1mm=0.73±0.10 kpc) versus the F444W stellar Re=0.97±0.01 kpc and the integrated 1 mm flux (0.99±0.03 mJy).
• Figure 4: Trace the slitless contamination geometry—identify the neighboring source (CDFS_00336) and its Brγ feature whose dispersed path overlaps S1—showing how this led to the earlier Hα misassignment at a single dispersion angle.