Discovery of new N-emitters over a wide redshift range

TL;DR

This study systematically identifies 45 N-emitters with UV nitrogen lines in public JWST NIRSpec data, spanning z ≈ 3–11 and a wide range of host galaxy properties. By combining UV and optical emission lines, the authors derive metallicities in the range $12 + \\log({\rm O/H}) \\approx 7.15-8.50$ and consistently find supersolar N/O and elevated N/C across metallicities, while C/O remains near normal. The results reveal diverse morphologies and stellar populations, including Balmer breaks indicative of composite ages, and show that the fraction of N-emitters increases with redshift, suggesting a link to early, dense star-forming environments or clustered star formation. These findings place strong constraints on nitrogen-enrichment scenarios, challenging carbon-rich WC-origin models and favor rapid, localized N production possibly in massive star-dominated clusters, with implications for galaxy evolution at the epoch of reionization. Follow-up spatially resolved spectroscopy will be key to pinpoint the sites and timescales of nitrogen enrichment and to test the proposed scenarios.

Abstract

JWST observations have revealed rare galaxies with UV spectra exhibiting intense lines of nitrogen, indicative of super-solar N/O abundances at low metallicity. To better understand these enigmatic objects and provide new constraints on proposed scenarios, we have undertaken a systematic search for galaxies with UV emission lines of nitrogen. Using public JWST NIRSpec data, we have identified 45 N-emitters with robust NIII] or NIV] detections, including 4 previously known objects. We find N-emitters from redshift $z\sim 3-11$ among a broad diversity of galaxies, in terms of morphology, UV magnitude, stellar mass, SFR, metallicity, and rest-optical line strengths. The UV nitrogen lines show typical equivalent widths between $\sim 5-50$ Å. Carbon lines are generally fainter than the N lines. Using strong line calibrations established at high-redshift, we find metallicities $12+log(O/H)\sim 7.15-8.5$, including thus also high metallicities. The H$β$ equivalent width 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 of the UV emission lines and an older population contributing to the rest-optical spectrum. Supersolar 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 high N/C ratios or lower limits ($\log(N/C) > 0.5$), independently of metallicity. The observed abundance ratios are compatible with ejecta from H-burning and do not show 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. Our study increases the sample of known N-emitters by a factor $\sim 3$, reveals a diversity of properties among N-emitters, and provides new constraints on their nature.

Figures (11)

• Figure 1: NIRSpec/PRISM spectra of two N-emitters, illustrating the diversity of our sample. The top spectrum corresponds to a highly star-forming galaxy with strong optical lines. The bottom one shows a Balmer break at $\sim 3740$Å, a very low EW(H$\beta$), and faint optical lines. Both show at least one significant UV nitrogen line.
• Figure 2: Top: Absolute UV magnitude _ UV $M_{\rm UV}$ of all known N-emitters (60 in total) versus redshift. The literature sample is marked with crosses. Colours distinguish objects categorised as SFGs (blue), LRDs (black), and AGN (red). The literature sample also includes four lensed galaxies, whose magnitudes have been corrected for gravitational lensing. Bottom: Mass--SFR showing a subset of the N-emitters (symbols colour-coded by redshift) for which the stellar mass could be derived, and the SF main sequence of 2025Cole at $z=7-9$ (dashed).
• Figure 3: Observed properties of N-emitters (SFG in blue, LRD in black, AGN in red). Equivalent widths of the UV lines (left: N iii] $\lambda$1750, N iv] $\lambda$1486, right: C iii] $\lambda$1909, C iv$\lambda$1550) are shown as a function of EW(β H$\beta$) in the top row. Typical errorbars are comparable to the size of the symbols, and therefore not shown. Upper limits for N iv] $\lambda$1486 and C iv$\lambda$1550 are shown as blue arrows, limits for N iii] $\lambda$1750 and C iii] $\lambda$1909 as magenta arrows. Observed correlations describing Carbon-emitters, from Tang2025The-JWST-Spectr are shown as dotted and dashed lines (for C iii] and C iv). Line ratios of N iv] $\lambda$1486/N iii] $\lambda$1750 versus EW(β H$\beta$), and O32 versus R23 are shown in the bottom panels. Low-$z$ and high-$z$ data from Izotov2021Low-redshift-co and DEugenio2025JADES-Data-Rele are shown in the bottom right panel. In all panels, $3 \sigma$ limits are shown. EWs are in the rest frame.
• Figure 4: Derived CNO abundance ratios of the N-emitters (colored symbols) as function of metallicity (O/H). Top left: N/O abundances computed for variable $T_e(O/H)$, top right: for fixed $T_e=22000$ K. Colored squares show N/O(UV) values from UV lines only (orange, magenta, and green for Eqs. \ref{['eq_nouv1']}, \ref{['eq_nouv1']}, \ref{['eq_nouv3']}) and red diamonds show N/O(UV-opt) derived from UV and optical lines following Eq. \ref{['eq_opt']}. Bottom left: N/C and lower limits from UV lines (blue), bottom right: C/O, using same colored symbols. Low-$z$ star-forming galaxies and H ii regions from the compilation of Izotov2023Abundances-of-C are shown by small black symbols, respectively. Dash-dotted lines show the average trend observed in low-$z$ star-forming galaxies, as parametrised by Vila-Costas1993The-nitrogen-to and Nicholls2017Abundance-scali respectively.
• Figure 5: NIRCam F115W + F356W + F444W composite $1\arcsec\times1\arcsec$ images of four N-emitters from our sample, illustrating the morphological diversity within the population, including compact, disky, and clumpy structures.