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Arxiv: New Methods of Identifying AGN in the Early Universe using Spectroscopy and Photometry in the JWST Era Published: 1/16/2025 5:10:26 PM Updated: 1/16/2025 5:10:26 PM

Paper abstract: We explore spectroscopic and photometric methods for identifyinghigh-redshift galaxies containing an Active Galactic Nucleus (AGN) with JWSTobservations. After demonstrating the limitations of standard optical methods,which appear ineffective in the low-metallicity environment of the earlyuniverse, we evaluate alternative diagnostic techniques using the current JWSTobservational capabilities. Our analysis focuses on line ratios and equivalentwidths (EWs) of UV emission lines: CIV, HeII \lambda1640, OIII]\lambda1665, and CIII], and the faint optical line, HeII \lambda4686. Wefind that the most valuable diagnostic quantities for finding AGN are the lineratios: (CIII] + CIV) / HeII \lambda1640 and CIII] / HeII \lambda1640, aswell as the EW of HeII \lambda1640. For more reliable AGN identification, theHeII \lambda1640 and OIII] \lambda1665 lines would need to be detectedseparately. We show that the HeII \lambda1640/H\beta ratio effectivelyseparates AGN from star-forming galaxies, though it is contingent on a low dustcontent. We also show that in order to effectively use these diagnostics,future observations require longer exposure times, especially for galaxies atz > 6. Subsequently, we plot three real high-redshift sources on thesediagrams which present strong UV emission lines. However, in order to classifythem as strong AGN candidates, further study is needed due to the blending ofHeII + OIII] and unreliable optical lines. Lastly, we carry out a selectionprocess using spectral energy distribution (SED) fitting with EAZY to identifystrong AGN candidates in the JADES NIRCam photometry. One galaxy in our sampleemerged as a strong AGN candidate, supported by both photometric selection andstrong UV emission. We present a sample of similar AGN candidates in the JADESdata based on this method.