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 $\lambda$1640, OIII] $\lambda$1665, and CIII], and the faint optical line, HeII $\lambda$4686. We find that the most valuable diagnostic quantities for finding AGN are the line ratios: (CIII] + CIV) / HeII $\lambda$1640 and CIII] / HeII $\lambda$1640, as well as the EW of HeII $\lambda$1640. For more reliable AGN identification, the HeII $\lambda$1640 and OIII] $\lambda$1665 lines would need to be detected separately. We show that the HeII $\lambda$1640/H$\beta$ 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.

submitted MNRAS, 21 pages

References in corpus (29)

• Array Programming with NumPy
• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• The Host Galaxies and Classification of Active Galactic Nuclei
• EAZY: A Fast, Public Photometric Redshift Code
• Binary Population and Spectral Synthesis Version 2.1: construction, observational verification and new results
• Cosmic Reionization and Early Star-Forming Galaxies: A Joint Analysis of New Constraints from Planck and Hubble Space Telescope
• The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope I. Overview of the instrument and its capabilities
• The James Webb Space Telescope Mission
• NIRCam Performance on JWST In Flight
• A CEERS Discovery of an Accreting Supermassive Black Hole 570 Myr after the Big Bang: Identifying a Progenitor of Massive z > 6 Quasars
• JADES. The diverse population of infant Black Holes at 4<z<11: merging, tiny, poor, but mighty
• Modelling the nebular emission from primeval to present-day star-forming galaxies
• GA-NIFS: A massive black hole in a low-metallicity AGN at $z\sim 5.55$ revealed by JWST/NIRSpec IFS
• The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope II. Multi-object spectroscopy (MOS)
• JADES NIRSpec Initial Data Release for the Hubble Ultra Deep Field: Redshifts and Line Fluxes of Distant Galaxies from the Deepest JWST Cycle 1 NIRSpec Multi-Object Spectroscopy
• Diagnostics for PopIII galaxies and Direct Collapse Black Holes in the early universe
• X-ray binaries as the origin of nebular HeII emission in low-metallicity star-forming galaxies
• Overview of the JWST Advanced Deep Extragalactic Survey (JADES)
• Spectral Templates Optimal for Selecting Galaxies at z > 8 with JWST
• JADES: A large population of obscured, narrow line AGN at high redshift
• High-z galaxies with JWST and local analogues -- it is not only star formation
• New AGN diagnostic diagrams based on the [OIII]$\lambda 4363$ auroral line
• EPOCHS VII: Discovery of high redshift ($6.5<z<12$) AGN candidates in JWST ERO and PEARLS data
• Nebular emission from young stellar populations including binary stars
• JADES: comprehensive census of broad-line AGN from Reionization to Cosmic Noon revealed by JWST
• The JADES Origins Field: A New JWST Deep Field in the JADES Second NIRCam Data Release
• Unveiling hidden active nuclei in MaNGA star-forming galaxies with HeII$\lambda$4686 line emission
• A $Li$ne $Me$asuring library for large and complex spectroscopic data sets: Implementation of a virtual observatory for JWST spectra
• Classifying Intermediate Redshift Galaxies in SDSS: Alternative Diagnostic Diagrams

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