The AGN nature of strong C III] emitters in the early Universe with JWST

¹ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00078 Monteporzio Catone, Italy
² Dipartimento di Fisica, Università di Roma Sapienza, Città Universitaria di Roma – Sapienza, Piazzale Aldo Moro, 2, 00185 Roma, Italy
³ Physics Department, Tor Vergata University of Rome, Via della Ricerca Scientifica 1, 00133 Rome, Italy
⁴ Institute of Physics, Laboratory for Galaxy Evolution and Spectral Modelling, EPFL, Observatoire de Sauverny, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁵ University of the Pacific, Stockton, CA 90340, USA
⁶ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁷ Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045, USA
⁸ Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
⁹ Institute for Computational and Data Sciences, The Pennsylvania State University, University Park, PA 16802, USA
¹⁰ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
¹¹ NSF’s NOIRLab, Tucson, AZ 85719, USA
¹² Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
¹³ Department of Astronomy, The University of Texas at Austin, Austin, TX, USA
¹⁴ Cosmic Frontier Center, The University of Texas at Austin, Austin, TX, USA
¹⁵ University of Massachusetts Amherst, 710 North Pleasant Street, Amherst, MA 01003-9305, USA
¹⁶ Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623, USA
¹⁷ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁸ INFN – Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy
¹⁹ Department of Physics, 196A Auditorium Road, Unit 3046, University of Connecticut, Storrs, CT 06269, USA
²⁰ School of Astronomy and Space Science, University of Chinese Academy of Sciences (UCAS), Beijing 100049, China
²¹ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
²² Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, China

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Received: 18 December 2025
Accepted: 9 March 2026

Abstract

The semi-forbidden C III] λλ1907,1909 doublet is a key tracer of high-ionisation emission in the early Universe. We present a study of C III] emission in galaxies at z = 5 − 7, using publicly available JWST/NIRSpec prism data from programmes including CEERS, JADES, and RUBIES, along with data from the ongoing CAPERS survey. We built a sample of 61 C III]-emitting galaxies, and we classified them as star-forming galaxies (SFGs) or active galactic nucleus (AGN) host galaxies using (1) rest-frame UV and optical emission-line diagnostic diagrams, and (2) the presence or absence of broad Balmer emission lines. The UV diagnostics are based on the combination of the rest-frame equivalent width (EW) of C III] versus the ratio C III]/He IIλ1640, and the EW of C IV versus the ratio C IV/He IIλ1640. For optical diagnostics, we used the OHNO diagram, which combines four optical emission lines that span a range in ionization potential: [O III] λ5008, Hβ, [Ne III] λ3869, and [O II] λλ3726,3729. We find that it separates AGNs from SFGs with low efficiency. Half of the sources in our sample (29 out of 61 galaxies) exhibit at least one secure indication of AGN activity, while 13 are potential AGNs based on the C III] diagnostic. We derived physical properties, including stellar mass and star formation rate, through spectral energy distribution modelling with BAGPIPES. Our analysis shows that JWST is uncovering a population of strong C III] emitters at high redshift (5 < z < 7) with a median EW of 22.8 Å (16th–84th percentile: 12.5–51.5 Å). This EW exceeds than that of a control sample of C III] emitters at intermediate redshift (3 < z < 4), which have a median EW of 4.7 Å. For the same range of M_UV, the C III] EW increases by ∼0.67 dex from 3 < z < 4 to 5 < z < 7, indicating strong redshift evolution in the line strength. We also find that C III] emitters span the full range of the star-forming main sequence, where the majority of galaxies below the sequence have a high C III] EW. Finally, we identify five sources in our sample as little red dots (LRDs) according to the selection criteria in the literature; four of these have already been identified as LRDs, while one is presented here for the first time.