Confirming Near- to Mid-IR Photometrically-Identified Obscured AGNs in the JWST era

George H. Rieke, Yang Sun, Jianwei Lyu, Christopher N. A. Willmer, Yongda Zhu, Pierluigi Rinaldi, Meredith A. Stone, Kevin N. Hainline, Pablo G. Perez-Gonzalez

First listed 2025-10-08 | Last updated 2025-10-08

Abstract

We evaluate the underlying assumptions for the identification of Active Galactic Nuclei (AGNs) through near- and mid-infrared photometry and spectral energy distribution (SED) fitting out to z ~ 3. For massive galaxies, log(M) > 9.5, our high resolution spectra of the rest optical range generally confirm the results of SED fitting, which relies primarily on excesses above the stellar emission between 1 and 6 microns to identify AGN. However, the method is undermined if the redshift used for the SED fitting is incorrect. Low mass galaxies, log(M) < 9:5, can contain relatively warm dust that emits in the 4 - 6 micron range. We show that the potential contamination of AGN samples by purely star forming low-mass galaxies can be avoided by the use of the infrared properties of Haro 11 as a limiting star-forming SED template. However, relatively few star forming galaxies emit as strongly in the 3 - 6 micron range as this template, so this could result in missing some obscured AGNs to avoid a minor contamination. Including the behavior of the galaxies at rest lamda ~ 13.5 microns can mitigate this problem and yield more complete samples of bona fide AGN. JWST/MIRI supports this approach out to z ~ 0.6.

Short digest

Tests how well near–mid-IR photometric SED fitting picks out obscured AGN by comparing to JWST/NIRSpec MSA rest‑optical spectra for 17 MIRI‑selected candidates at z=1.24–3.33, and by auditing low‑mass interlopers. For massive hosts (log M*>9.5), the spectra and BPT diagnostics generally back the 1–6 μm excess criterion, with several sources showing outflow‑broadened [O III]/Balmer lines. Low‑mass galaxies can mimic a 4–6 μm AGN excess via warm dust; adopting Haro 11 as a limiting star‑forming SED curbs contamination but can miss some obscured AGN. Adding rest λ≈13.5 μm behavior restores completeness, feasible with MIRI out to z≈0.6, while emphasizing that wrong redshifts can undermine SED‑based IDs.

Key figures to inspect

• Figure 1 (BPT placement): Check where the 17 MIRI‑selected candidates land relative to Kewley/Kauffmann lines to see spectroscopic confirmation of obscuration and which lie in star‑forming vs AGN zones.
• Figure 1 (example spectra A=87191, B=209962, C=196290): Inspect [O III] and Balmer line widths to separate outflow‑broadened narrow lines from genuine broad‑line components and link to the obscuration implied by the SEDs.
• Figure 2 (SED decompositions): Verify that the AGN component is required by the 1–6 μm excess across acceptable fits, note how redshift shifts PAH features and drives degeneracy, and check how adding rest ~13.5 μm constrains the solution.
• Figure 2 (continued panels): Compare massive vs dwarf hosts to see when a warm, non‑PAH stellar IR excess can masquerade as AGN and how the fit partitions flux among stellar, hot‑dust (star‑forming), and AGN components.
• Figure 4 (WISE colors of dwarfs): Gauge how local low‑metallicity dwarfs intrude into AGN color wedges, and use Haro 11’s track as a conservative boundary to balance completeness vs contamination.