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

TL;DR

This study leverages JWST/NIRSpec spectroscopy of 16 ALMA-detected faint SMGs in the HUDF to measure rest-frame optical emission lines and derive ISM properties, metallicities, and AGN activity across $z\sim1$--$4$. By combining JWST spectra with deep multiwavelength photometry and Chandra X-ray data, the authors show a high incidence of obscured AGN activity in massive SMGs ($M_* > 10^{10.5} M_\odot$), with metallicities in the range $\sim0.4$--$2\,Z_\odot$ and electron densities $n_e\sim10^2$--$10^3\ \mathrm{cm^{-3}}$, consistent with typical high-$z$ star-forming galaxies. The gas-phase metallicities place these systems above the critical metallicity for efficient ISM grain growth, naturally explaining their dusty nature, while they largely follow the SFR–$M_*$ and mass–metallicity relations. The results support a view in which faint SMGs are representative of massive high-redshift galaxies with co-evolving star formation and SMBH growth, rather than a distinctly separate population, and highlight the role of metallicity in driving dust mass buildup. Overall, the work demonstrates the power of JWST to characterize the dust-enshrouded ISM and nuclear activity in the early universe through rest-frame optical spectroscopy.

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.

Figures (18)

• Figure 1: Relation between IR luminosity and redshift. Dust-continuum sources in HUDF are shown as open red circles (UDF: dunlop17; ASPECS: aravena20). Filled red circles indicate ALMA sources with JWST/NIRSpec (UDF+ASPECS sample in this study). The black crosses mark X-ray detections luo17. Comparison samples are local ULIRGs at $z<0.5$dacunha10, ALESS hodge13dacunha15, JWST/NIRCam dark galaxies at $z=6.6$sun25, and REBELS bouwens22rowland25. Some high-redshift bright SMGs are also highlighted as magenta (SPT0311-58: e.g., marrone18, HFLS3: e.g., riechers13, HZ10: e.g., capak15, SPT0418-47: e.g., cathey24). The black solid and dotted curves show the $L_{\rm IR}$ corresponding to the ASPECS ($S_{\rm 1.2\,mm}>0.03\,{\rm mJy}$; $3.3\sigma$) and HUDF ($S_{\rm 1.3\,mm}>0.12\,{\rm mJy}$; $3.5\sigma$) detection limits, respectively, assuming dust temperature of $T_{\rm dust}=40$ K and emissivity index of $\beta_{\rm IR}=1.5$.
• Figure 2: UDF dunlop17 and ASPECS aravena20 sky regions and the dust continuum source positions overlaid on the JWST/NIRCam F444W image. The blue and orange regions represent the survey areas of UDF and ASPECS, respectively. ALMA dust-continuum sources reported in the UDF and the ASPECS are shown as black open circles. The red open circles indicate the UDF or ASPECS sources with JWST/NIRSpec (UDF+ASPECS sample in this study).
• Figure 3: Relation between UV absolute magnitude ($M_\mathrm{UV}$) and 1 mm flux density ($S_\mathrm{1\,mm}$). Dust-continuum sources reported in the UDF survey dunlop17 and the ASPECS survey aravena20 are shown as open circles (parent sample). Filled red circles indicate the sources with JWST/NIRSpec in the parent sample (UDF+ASPECS sample). If sources are detected in both ASPECS (1.2 mm) and UDF (1.3 mm), we plot the 1-mm flux density reported in the ASPECS aravena20. Sources with non-detections in the rest-frame UV are shown as $3\sigma$ lower limits on $M_\mathrm{UV}$ as arrows. The black and red double diamonds show the medians for the parent sample and the UDF+ASPECS sample, respectively. The associated error bars indicate the 16th–84th percentile ranges. The black solid and dotted lines show the ASPECS ($S_{\rm 1.2\,mm}>0.03~{\rm mJy}$; $3.3\sigma$) and UDF ($S_{\rm 1.3\,mm}>0.12~{\rm mJy}$; $3.5\sigma$) detection limits, respectively.
• Figure 4: Relation between star formation rate (SFR) and stellar mass. The red circles show the UDF+ASPECS sample. The X-ray detected sources and BPT-AGN sources (Section \ref{['subsec:AGN-SMGs']}) are shown as the black crosses and black squares, respectively. The magenta double diamonds with errorbars show the median and the 16th–84th percentiles of the UDF+ASPECS sample in two stellar-mass bins ($M_*<10^{10.4}~M_\odot$ and $M_*>10^{10.4}~M_\odot$). The red line shows the star formation main sequence at $z\sim2$santini17. The ALESS dacunha15, the REBELS rowland25, and the local ULIRGs dacunha10 are also shown.
• Figure 5: Some example SED-fitting results (left: C08 as an example of an AGN; right: C13 as prominent PAH emission). Top: The purple squares and green triangles indicate the observed photometry and the $3\sigma$ upper limits, respectively. The black curve shows the total best-fit SED, while the red, orange, blue-dashed, and yellow curves show the dust emission, AGN emission, dust-unattenuated, and dust-attenuated stellar emission, respectively. The reduced $\chi$-square of the best-fit SED is indicated above the panels. Bottom: Relative residuals, $(F_\mathrm{obs}-F_\mathrm{model})/F_\mathrm{obs}$, between the observed photometry and the total best-fit SED. We note that AGN fractions ($f_\mathrm{AGN}$) of C08 and C13 are $0.50\pm0.03$ and $0.01\pm0.02$, respectively.