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

TL;DR

This study uses JWST/NIRSpec near-infrared spectroscopy to identify dust-obscured ionizing sources in galaxies at redshifts 1 to 3, revealing a small population of dominant dust-obscured AGN missed by optical diagnostics.

Contribution

It introduces near-infrared emission line diagnostics for classifying ionizing sources in dusty galaxies, highlighting their effectiveness over optical methods.

Findings

~90% of classifications are consistent across optical and near-infrared wavelengths.

Identification of obscured AGN missed by optical diagnostics.

Evidence of dust-obscured star formation and elevated N/Fe in some sources.

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.