2024A&A...691A.345M

The James Webb Space Telescope (JWST) is revolutionizing our understanding of black hole formation and growth in the early Universe. However, JWST has also revealed that some of the classical diagnostics, such as the Baldwin, Phillips & Terlevich (BPT) diagrams and X-ray emission, often fail to identify active galactic nuclei (AGN) at high redshift and low metallicity. Here we present three new rest-frame optical diagnostic diagrams to identify narrow-line Type II AGN, leveraging the [OIII]λ4363 auroral line, which has been detected in several JWST spectra. Specifically, we show that high values of the [OIII]λ 4363 / Hγ ratio provide a sufficient (but not necessary) condition to identify the presence of an AGN, based on empirical calibrations (using local and high-redshift sources) and on a broad range of photoionization models. These diagnostics are able to separate much of the AGN population from star-forming galaxies (SFGs): the average energy of an AGN's ionizing photons is higher than that of young stars in SFGs, and hence AGN can more efficiently heat the gas, thus boosting the [OIII]λ4363 line. We also found independent indications of AGN activity in some high-redshift sources (z > 4) that were not previously identified as AGN with the traditional diagnostics diagrams, but that are placed in the AGN region of the diagnostic presented in this work. We note, conversely, that low values of [OIII]λ 4363 / Hγ can be associated either with SFGs or AGN excitation. We note that the fact that strong auroral lines are often associated with AGN does not imply that they cannot be used for direct metallicity measurements (provided that proper ionization corrections are applied), but it does affect the calibration of strong line metallicity diagnostics.