Narrow-line AGN selection in CEERS: Spectroscopic selection, physical properties, and X-ray and radio analysis

¹ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, I-40129 Bologna, Italy
² INAF –Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy
³ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB30HA, UK
⁴ Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁵ Department of Physics and Astronomy, University College London, Gower Street, London WC1E6BT, UK
⁶ Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy

^⋆ Corresponding author: giovanni.mazzolari@inaf.it

Received: 11 August 2024
Accepted: 7 February 2025

Abstract

The transformative era opened by the James Webb Space Telescope (JWST) on the high-z Universe allows us to investigate the early stages of supermassive black hole (SMBH) evolution, with the first results showing a greater than expected number of active galactic nuclei (AGNs) at very early times. In this work, we spectroscopically select narrow-line AGNs (NLAGNs) among the ∼300 publicly available medium-resolution spectra of the Cosmic Evolution Early Release Science Survey (CEERS). Using both traditional and newly identified emission line NLAGN diagnostics diagrams, we identified 52 NLAGNs at 2 ≲ z ≲ 9, on which we performed a detailed multiwavelength analysis. We also identified four new z ≲ 2 broad-line AGNs (BLAGNs), in addition to the eight previously reported z > 4.5 BLAGNs. We found that the traditional BPT diagnostic diagrams are not suited to identifying high-z AGNs, while most of the high-z NLAGN were selected using the recently proposed AGN diagnostic diagrams based on the [O III] λ4363 auroral line or high-ionization emission lines. We compared the emission line velocity dispersion and the obscuration levels of the sample of NLAGNs with those of the parent sample without finding significant differences between the two distributions, suggesting a population of AGNs heavily buried and not significantly impacting the host galaxies’ physical properties, as was further confirmed by spectral energy distribution fitting. The bolometric luminosities of the high-z NLAGNs selected in this work are ∼1.5 dex below the ones sampled by surveys before JWST, potentially explaining the weak impact of these AGNs. Finally, we investigated the X-ray properties of the selected NLAGNs and of the sample of high-z BLAGNs. We found that all but four NLAGNs are undetected in the deep X-ray image of the field, as well as all the high-z BLAGNs. We did not obtain a detection even by stacking the undetected sources, resulting in an X-ray weakness of ∼1 − 2 dex from what was expected based on their bolometric luminosities. To discriminate between a heavily obscured AGN scenario or an intrinsic X-ray weakness of these sources, we performed a radio (1.4GHz) stacking analysis, which did not reveal any detection and left open the questions about the origin of the X-ray weakness.