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

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$\alpha$, [N II]$\lambda \lambda 6548,6583$, and [S II]$\lambda \lambda 6717,6731$) and faint shorter-wavelength optical lines (H$\beta$ and [O III]$\lambda \lambda 4959,5007$) with a large nebular attenuation, $E(B-V)\sim 0.3$-$1.8$. 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 $\sim 80\%$ for the SMGs at $M_{\ast }>10^{10.5}{M}_{\odot }$. 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, $\sim 0.4$-$2Z_{\odot }$, exceeding the model-predicted dust-growth critical metallicity ($\sim 0.1$-$0.2Z_{\odot }$), 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 $n_e\sim 10^2$-$10^3{\mathrm{cm}}^{-3}$ for the SMGs that are comparable with other high-$z$ galaxies. Together with the high SMG fraction ($\sim 100\%$) at the massive end ($M_{\ast }>10^{10.5}{M}_{\odot }$), these results indicate that the SMGs are mostly not special, but typical massive star-forming galaxies at high redshift.

29 pages, 18 figures, 4 tables. Submitted to ApJ. Comments are welcome