JWST Spectroscopic Census of ALMA Faint Submillimeter Galaxies in the Hubble Ultra Deep Field

Tomokazu Kiyota, Masami Ouchi, Daisuke Iono, Seiji Fujimoto, Kotaro Kohno, Yoshihiro Ueda, Kimihiko Nakajima, Moka Nishigaki, Hidenobu Yajima

First listed 2026-01-26 | Last updated 2026-01-26

Abstract

We present a JWST/NIRSpec rest-frame optical spectroscopic census of ALMA 1-mm continuum sources in the Hubble Ultra Deep Field (UDF) identified by the deep ALMA UDF and ASPECS programs. Our sample is composed of the ALMA flux-limited ($S_{1\,\mathrm{mm}}\gtrsim 0.1\,\mathrm{mJy}$) sources observed with medium-resolution NIRSpec spectroscopy from JADES and SMILES, 16 faint submillimeter galaxies (SMGs) at spectroscopic redshifts of $z\sim 1$-$4$. These SMGs show bright longer-wavelength optical lines (H$α$, [N II]$λ\lambda6548,6583$, and [S II]$λ\lambda6717,6731$) and faint shorter-wavelength optical lines (H$β$ and [O III]$λ\lambda4959,5007$) with a large nebular attenuation, $E(B-V)\sim0.3$-$1.8$. We test the SMGs using BPT diagnostics and Chandra X-ray fluxes, and find that most SMGs are classified as AGNs; the AGN fraction is $\sim80\%$ for the SMGs at $M_*>10^{10.5} M_\odot$. We find only one SMG ($<10\%$) with a broad Balmer line, indicating that the SMGs are predominantly obscured AGNs. With the optical lines, we estimate the metallicities of the SMGs to be moderately high, $\sim0.4$-$2 Z_\odot$, exceeding the model-predicted dust-growth critical metallicity ($\sim0.1$-$0.2Z_\odot$), which naturally explains the dusty nature of the SMGs. Interestingly, the SMGs fall in the mass-metallicity relation and the star-formation main sequence, showing no significant differences from other high-$z$ galaxies. Similarly, we find electron densities of $n_e\sim10^2$-$10^3\,\mathrm{cm}^{-3}$ for the SMGs that are comparable with other high-$z$ galaxies. Together with the high SMG fraction ($\sim 100\%$) at the massive end ($M_*>10^{10.5} M_\odot$), these results indicate that the SMGs are mostly not special, but typical massive star-forming galaxies at high redshift.

Short digest

A medium-resolution JWST/NIRSpec census targets 16 ALMA 1‑mm–selected HUDF sources (S1mm ≥ 0.1 mJy) at z ≈ 1–4 from JADES/SMILES, delivering rest‑optical line diagnostics. The spectra show bright Hα+[N II]+[S II] but faint Hβ+[O III], implying heavy nebular attenuation E(B−V) ≈ 0.3–1.8; BPT plus Chandra indicate most are AGN, with ~80% AGN fraction at M* > 10^10.5 M⊙ and only one broad‑line case, pointing to predominantly obscured nuclei. Metallicity is moderately high (~0.4–2 Z⊙), above dust-growth thresholds (≈0.1–0.2 Z⊙), with electron densities n_e ≈ 10^2–10^3 cm⁻3 comparable to other high‑z systems. These SMGs sit on the mass–metallicity relation and the star-forming main sequence, and—together with the near‑unity massive‑end SMG incidence—appear to be typical massive star-forming galaxies rather than special outliers.

Key figures to inspect

• Figure 1 (L_IR–z coverage): Verify the sample’s placement relative to ALMA detection limits and note which sources have X-ray counterparts, clarifying luminosity range and selection completeness across z≈1–4.
• Figure 2 (HUDF/ASPECS footprint on F444W): Check which ALMA detections received NIRSpec MSA slits and their locations within the deep imaging/X-ray fields, to assess environmental and ancillary-data leverage.
• Figure 3 (M_UV vs S_1mm): Inspect how UV faint the ALMA sources are (including lower limits) and compare medians of the parent vs NIRSpec subsample to gauge any dust/brightness bias in the spectroscopic census.
• Figure 4 (SFR–M* plane): Confirm that SMGs lie on the star-forming main sequence and see whether BPT/X-ray AGN hosts deviate, constraining how nuclear activity coexists with typical star formation.