New methods of identifying AGN in the early Universe using spectroscopy and photometry in the <i>JWST</i> era

ABSTRACT We explore spectroscopic and photometric methods for identifying high-redshift galaxies containing an active galactic nucleus (AGN) with James Webb Space Telescope (JWST) observations. After demonstrating the limitations of standard optical methods, which appear ineffective in the low-metallicity environment of the early universe, we evaluate alternative diagnostics using current JWST capabilities. Our analysis focuses on line ratios and equivalent widths (EWs) of ultraviolet (UV) lines: C iv, He ii $\lambda$1640, O iii] $\lambda$1665, and C iii], and the faint optical line He ii $\lambda$4686. We find that the most valuable diagnostic quantities for identifying AGN are the ratios (C iii] + C iv)/He ii $\lambda$1640 and C iii]/He ii $\lambda$1640, as well as the EW of He ii $\lambda$1640. For more reliable AGN identification, He ii $\lambda$1640 and O iii] $\lambda$1665 should be detected separately. The He ii $\lambda$1640/H$\beta$ ratio effectively separates AGN from star-forming galaxies, although it depends on low dust content. We also show that to use these diagnostics effectively, future observations require longer exposure times, especially at z > 6. We plot three high-redshift sources with strong UV emission on these diagrams; however, further study is needed to classify them as AGN candidates due to He ii + O iii] blending and unreliable optical lines. Finally, we select AGN candidates in the JWST Advanced Deep Extragalactic Survey (JADES) Near Infrared Camera photometry using spectral energy distribution fitting with eazy. One galaxy emerges as a strong AGN candidate, supported by both photometric selection and strong UV emission, and we present a sample of similar candidates in the JADES data using this method.