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

TL;DR

This paper assesses the reliability of photometric near- to mid-infrared AGN identifications in the JWST era by testing the assumptions behind PAH-based SED fitting and addressing potential contamination from hot-dust emission in low-mass galaxies. It uses JWST/NIRSpec MSA spectroscopy of 17 MIRI-selected obscured AGN candidates from the SMILES survey to compare spectroscopic diagnostics with photometric classifications and to derive metallicities. The authors show that low-mass galaxies with warm dust can mimic AGN signatures, but adopting an Haro 11-like star-forming SED template and incorporating rest-frame 13.5 μm information reduces contamination and yields more complete samples, albeit with some risk of missing certain obscured AGN. The study demonstrates a viable pathway for robust AGN censuses in JWST surveys, highlighting the role of spectroscopy in validating photometric selections, and indicating applicability up to moderate redshifts (with photometric tests extending to z ~ 0.6).

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.