JWST/NIRSpec Reveals a Small Population of Dominant Dust-Obscured Ionizing Sources in Galaxies at 1 < z < 3

Si-Rui Ge, Nikko J. Cleri, Joel Leja, Antonello Calabro, Vital Fernandez

First listed 2025-11-11 | Last updated 2025-11-11

Abstract

Rest-frame optical emission line diagnostics are often used to help classify ionizing sources within galaxies. However, rest-frame optical tracers can miss sources with high dust attenuation, leading to misclassification of the dominant ionizing source. Longer wavelength tracers, such as those in the near-infrared, carry the power to diagnose ionizing sources while being more robust than optical tracers to the presence of dust. The diagnostics used in this work employ the ratios of bright near-infrared emission lines [S III] 9530, [Fe II] 12566 and [Fe II] 16443 to Paschen lines in publicly-available JWST/NIRSpec MSA medium-resolution spectroscopy of 55 galaxies at z < 3. We compare the rest-frame near-infrared and rest-frame optical diagnostics and find that ~90% of our sample have consistent classifications across wavelengths (49/55), while the remaining sources can be explained through ionizing radiation obscured by dust and/or elevated N/Fe abundances. We identify three objects classified as star-forming in the rest frame optical and as active galactic nuclei (AGN) in the rest-frame near-infrared, which we interpret as obscured AGN. We also identify three objects which are classified as AGN in the rest-frame optical and star forming in the rest-frame near-infrared. We interpret two of these objects as AGN with obscured star formation and the other with elevated N/Fe. We discuss how future spatially-resolved and/or mid-infrared spectroscopy can test the relative contributions of AGN and stars to the ionizing photon budgets of these disagreeing sources.

Short digest

Public NIRSpec MSA medium-resolution spectra are used to test near-IR line-ratio diagnostics—[S III] 9530, [Fe II] 12566, [Fe II] 16443 to Paschen lines (Fe2S3- and Fe2S3-)—against the optical [N II]-BPT for 55 galaxies at 1<z<3. About 90% (49/55) yield consistent classifications, but a small, revealing tail disagrees: three systems appear star-forming in the optical yet AGN in the near-IR (interpreted as dominant, dust-obscured AGN), while three show the reverse, attributed to obscured star formation in two cases and elevated N/Fe in one. The result shows that near-IR ratios robust to dust can recover dominant ionizing sources missed by optical tracers. This motivates spatially resolved and mid-IR follow-up to apportion AGN versus stellar contributions in the discrepant cases.

Key figures to inspect

• Figure 3: Compare positions in [N II]-BPT versus Fe2S3- and Fe2S3- planes; identify the six discordant objects and note which carry X-ray or mid-IR flags, testing the obscuration/abundance interpretations.
• Figure 4: Inspect Pa-to-Balmer decrement trends and the non–Case B shaded region to see which sources demand high attenuation or non-standard recombination, linking dust to the optical–near-IR classification flips.
• Figure 1: Check representative 1D spectra for secure detections and deblending ([N II]/Hα with M gratings; [S III], [Fe II], and Paschen lines), validating the S/N that anchors the near-IR ratios—especially [Fe II] 1.644 versus 1.257 μm coverage.
• Figure 2: Use the stacked redshift distribution (color-coded by diagnostic availability) to see where each line set falls in the NIRSpec setups and why certain diagnostics are accessible at particular z.