New Methods of Identifying AGN in the Early Universe using Spectroscopy and Photometry in the JWST Era

Abstract

We explore spectroscopic and photometric methods for identifying high-redshift galaxies containing an Active Galactic Nucleus (AGN) with 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 diagnostic techniques using the current JWST observational capabilities. Our analysis focuses on line ratios and equivalent widths (EWs) of UV emission lines: CIV, HeII $λ$1640, OIII] $λ$1665, and CIII], and the faint optical line, HeII $λ$4686. We find that the most valuable diagnostic quantities for finding AGN are the line ratios: (CIII] + CIV) / HeII $λ$1640 and CIII] / HeII $λ$1640, as well as the EW of HeII $λ$1640. For more reliable AGN identification, the HeII $λ$1640 and OIII] $λ$1665 lines would need to be detected separately. We show that the HeII $λ$1640/H$β$ ratio effectively separates AGN from star-forming galaxies, though it is contingent on a low dust content. We also show that in order to effectively use these diagnostics, future observations require longer exposure times, especially for galaxies at $z > 6$. Subsequently, we plot three real high-redshift sources on these diagrams which present strong UV emission lines. However, in order to classify them as strong AGN candidates, further study is needed due to the blending of HeII + OIII] and unreliable optical lines. Lastly, we carry out a selection process using spectral energy distribution (SED) fitting with EAZY to identify strong AGN candidates in the JADES NIRCam photometry. One galaxy in our sample emerged as a strong AGN candidate, supported by both photometric selection and strong UV emission. We present a sample of similar AGN candidates in the JADES data based on this method.

Figures (14)

• Figure 1: The NIRSpec spectra of galaxies 9422, 18846, and 10058975 are shown with spectral flux in units of $\mathrm{erg/s/cm^{2}/\text{\AA}}$ on the y axis and wavelength in units of angstroms on the x axis. The grey dashed lines are important emission lines found in the spectra. These spectra are from the JADES survey (p1_bunker2023jades).
• Figure 2: N2-BPT (on the left) and S2-VO87 (on the right) diagrams displaying photoionization models of active galactic nuclei (AGN, orange) and star-forming galaxies (SFG, blue) created by p11_NakajimaMaiolino2022 and various AGN sources from the literature (p4_maiolino2023jades, Harikane2023, Ubler_2023, Kocevski2023). The solid lines are extreme starburst lines defined by p3_kewley2001 and the dashed classification line is from Kauffman_2003. Additionally, the grey triangles present JADES data points from the R=1000 grating catalogue (p1_bunker2023jades). AGN are identified above the solid and dashed lines. A source in the Prism/Clear data catalogue with NIRSpec ID 16745, which contains all the necessary lines with a minimum SNR of 5, is located above the maximum starburst line on the S2-VO87 diagram. p14_scholtz2023jades selected this source as an AGN using similar considerations.Narrow-line fluxes were used to create these plots for all sources, except for source 16745, where the total flux was used.
• Figure 3: The OHNO diagram displaying SFG and AGN models from p11_NakajimaMaiolino2022 (coloured as previously), the solid and dashed black demarcation lines are based on the work of Backhaus_2022 and Feuillet_2024, respectively. Our newly defined solid red separation curve is defined by Equations \ref{['eq:OHNO_1']} and \ref{['eq:OHNO_2']}, and it separates a mixed region from a region only occupied by AGN models. Broad line AGN collected from literature (Larson_2023, Kokorev_2023, Kocevski2023), as well as sources from the JADES Prism/Clear and R=1000 grating data catalogues can be seen. Narrow line fluxes were used to create this plot with the exceptions of the data points from Larson_2023 and the JADES PRISM where the total line flux was used.
• Figure 4: Ultraviolet emission line flux ratio diagrams created based on the models from p11_NakajimaMaiolino2022 presented in Table \ref{['tab:model_parameters']}. As in Fig. \ref{['fig:standard_diagrams']}, the AGN models are represented with orange dots and the SF galaxy models with blue dots. The lighter blue points represent the SF galaxies models with very low metallicities (0.0007 Z$_{\odot}$ and 0.007 Z$_{\odot}$). These metallicities are 2--3 orders of magnitude lower than metallicities of AGN models in the same region. Our galaxies of interest (with NIRSpec IDs 9422, 18846, and 10058975) are represented using three different colors (purple, green and red).
• Figure 5: The EWs of the HeII $\lambda$1640 and HeII $\lambda$1640 + OIII] lines can be seen plotted against the emission line flux ratios CIII]/HeII $\lambda$1640 and CIII]/(HeII $\lambda$1640 + OIII]) blend. The plot to the right was created by replacing the HeII $\lambda$1640 line with the blended HeII $\lambda$1640 + OIII] line. The blending of HeII $\lambda$1640 and OIII] in the NIRSpec prism data reduces the diagnostic power of HeII $\lambda$1640 as an AGN indicator, except for AGN with the highest HeII $\lambda$1640 EW.