Discovery of new N-emitters over a wide redshift range

Context. Recent James Webb Space Telescope (JWST) observations have revealed a small number of galaxies with UV spectra exhibiting intense nitrogen emission lines, which are not typically detected in galaxy spectra. The observations indicate super-solar N/O abundances at low metallicity, a phenomenon whose nature is currently intensely debated. Aims. To better understand these enigmatic objects and provide new constraints on proposed scenarios, we did a systematic search for galaxies with UV nitrogen emission lines (N IV ] λ 1486 and/or N III ] λ 1750). Methods. Using public JWST NIRSpec data, we identified 45 N-emitters with robust N III ] or N IV ] detections, including four previously known objects. We then classified these objects, determined relative C, N, O, and H chemical abundance ratios, determined other properties, and carried out a statistical analysis of the N-emitter population. Results. We find N-emitters at redshifts z ∼ 3 − 11 spanning a broad diversity of galaxies in terms of morphology, UV magnitude, stellar mass, star formation rate, metallicity, and rest-optical emission line strengths. The UV nitrogen lines show typical equivalent widths (EW) between ∼5 Å and up to ∼100 Å in a few cases. Diverse ionisation conditions, as traced by N IV ] λ 1486/N III ] λ 1750, are observed. The carbon lines (C IV λ 1550 and C III ] λ 1909) are generally fainter than the N lines. Using strong line calibrations established at high-redshift, we find metallicities 12 + log(O/H)∼7.15 − 8.5, spanning up to high metallicity values. The H β EW of N-emitters varies strongly, and sources with low EWs show clear signs of a Balmer break, indicative of composite stellar populations combining both young (≲10 Myr) stars responsible for the UV emission lines and an older population contributing to the rest-optical spectrum. Super-solar N/O ratios are found in all N-emitters. C/O abundances are comparable to those of galaxies at the same metallicity, and all N-emitters show either high N/C ratios or lower limits (log(N/C) ≳ 0.5), regardless of metallicity. The observed abundance ratios are compatible with ejecta from H-burning and show no signs of carbon enhancements, even at higher metallicities. Finally, we find that the fraction of N-emitters increases with redshift, and we quantify this evolution. Conclusions. Our study increases the sample of known N-emitters by a factor of ∼3, reveals a diversity of properties among N-emitters, and provides new constraints on their nature.