2025ApJ...994...35R

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 μm 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 μm 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 μm 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 λ ∼ 13.5 μm can mitigate this problem and yield more complete samples of bona fide AGN. JWST/MIRI supports this approach out to z ≲ 0.6.