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

The semi-forbidden C III ] λλ 1907,1909 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 II λ 1640, and the EW of C IV versus the ratio C IV /He II λ 1640. For optical diagnostics, we used the OHNO diagram, which combines four optical emission lines that span a range in ionization potential: [O III ] λ 5008, H β , [Ne III ] λ 3869, and [O II ] λλ 3726,3729. 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 B AGPIPES . 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.