The AGN nature of strong CIII emitters in the Early Universe with JWST

F. Arevalo-Gonzalez, R. Tripodi, M. Llerena, L. Pentericci, A. Plat, G. Barro, R. O. Amorín, B. Backhaus, A. Calabrò, N. J. Cleri, M. Dickinson, J. S. Dunlop, S. L. Finkelstein, M. Giavalisco, M. Hirschmann, J. Kartaltepe, A. M. Koekemoer, R. A. Lucas, L. Napolitano, E. Piconcelli, A. J. Taylor, F. Tombesi, J. R. Trump, X. Wang

First listed 2025-12-18 | Last updated 2025-12-18

Abstract

The semi-forbidden CIII] $λλ$1907,1909 doublet is a key tracer of high-ionization emission in the early universe. We present a study of CIII] emission in galaxies at z=5-7, using publicly available JWST/NIRSpec prism data from programs including CEERS, JADES, RUBIES and CAPERS. We built a sample of 61 CIII]-emitting galaxies, and we classified them as star-forming or active galactic nuclei (AGN) host galaxies using (1) rest-frame UV and optical emission-line diagnostic diagrams, and (2) the presence/absence of broad Balmer emission lines. The UV diagnostics are based on the combination of the rest-frame equivalent width (EW) of CIII] versus CIII]/HeII $λ$1640, and the EW of CIV versus CIV/HeII $λ$1640. For optical diagnostics, we employ the OHNO diagram, which combines [OIII] $λ$5007, H$β$, [NeIII] $λ$3869, and [OII] $λλ$3727,3729- and we find it has a low efficiency on separating AGN from SFG. We find that 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 CIII] diagnostic. Physical properties, including stellar mass and star formation rate, are derived through spectral energy distribution modeling with Bagpipes. Our analysis reveals that JWST is uncovering a population of strong CIII] emitters at high redshift (5<z<7) with a median EW of 22.8 A. This EW is higher than that of a control sample of CIII] emitters at redshift 3<z<4 with a median EW of 4.7 A. We find that for the same range of Muv, the CIII] EW increases by $\sim$0.67 dex from 3<z<4 to 5<z<7, indicating strong redshift evolution in the line's strength. Finally, we identify five sources in our sample as Little Red Dots (LRDs); while four of these have already been identified as LRD in the literature, one is presented here for the first time.

Short digest

Builds a z=5–7 sample of 61 CIII] emitters from NIRSpec/prism (CEERS, JADES, RUBIES, CAPERS) and classifies them with UV-line diagnostics (EW(CIII])–CIII]/HeII; EW(CIV)–CIV/HeII) plus checks for broad Balmer lines, finding the OHNO optical diagram has low AGN/SF separation power. They report 29/61 with at least one secure AGN indicator and 13 additional AGN candidates from the CIII]-based diagnostic, and identify five Little Red Dots (one newly reported). The population shows strong CIII] with median EW 22.8 Å (12.5–51.5 Å), and at fixed MUV the line is stronger by ~0.67 dex than a 3<z<4 control (median 4.7 Å), implying rapid evolution. CIII] emitters span the star-forming main sequence, with the highest EWs common below the sequence, underscoring the utility of UV diagnostics for the early AGN census while cautioning against OHNO at these redshifts/resolution.

Key figures to inspect

• UV diagnostic planes: EW(CIII]) vs CIII]/HeII and EW(CIV) vs CIV/HeII with AGN/SF demarcations—inspect where the 29 secure AGN and 13 candidates land and where LRDs cluster.
• OHNO diagram ([OIII]/Hβ vs [NeIII]/[OII])—verify the claimed overlap of AGN and SFG loci and quantify the misclassification rate relative to UV diagnostics.
• EW(CIII]) vs MUV (and/or histogram) with the z=3–4 control overlay—check the median 22.8 Å and the ~0.67 dex EW offset at fixed MUV.
• Example NIRSpec/prism spectra for AGN-flagged sources—look for broad Balmer components (e.g., Hβ, Hγ) and note measured FWHM supporting AGN classification.
• SFR–M* plane from Bagpipes fits—confirm that CIII] emitters span the main sequence and that the largest EWs concentrate below the sequence.