Nitrogen abundances in star-forming galaxies 2.2 Gyr after the Big Bang are not elevated

TL;DR

This study uses deep JWST/NIRSpec rest-optical spectra of LyC22 galaxies at $z\sim3$ to derive nebular abundances via the direct method for 25 objects. The measured $\\log({\rm N/O}) = -1.29^{+0.25}_{-0.22}$ and $12+\log({\rm O/H})$ ranging from 7.56 to 8.44 show nitrogen abundances that are not elevated compared to local H II regions, indicating no strong redshift evolution of N/O for typical galaxies over a substantial fraction of cosmic time. The Ne/O ratio is $\\log({\rm Ne/O}) = -0.73^{+0.11}_{-0.08}$ and the galaxies display BPT offsets similar to $z\sim2.3$ systems, with the offset not caused by enhanced nitrogen. These results constrain the chemical evolution of nitrogen and oxygen at high redshift and establish a baseline for comparing to even earlier epochs ($z>4$) as larger samples become available.

Abstract

Using deep medium-resolution JWST rest-optical spectra of a sample of typical star-forming galaxies (Lyman break galaxies and Lyman-$α$ emitters) from the LyC22 survey at $z \sim 3$, we determined the nebular abundances of N, O, and Ne relative to H for a subsample of 25 objects with the direct method, based on auroral [OIII]4363 line detections. Our measurements increases the number of accurate N/O determinations at $z \sim 2-4$ using a homogeneous approach. We found a mean value of $\log({\rm N/O})=-1.29^{+0.25}_{-0.21} $ over a metallicity range 12+log(O/H)=7.5 to 8.44. The observed N/O ratio and scatter are indistinguishable from that observed in low-z galaxies and HII regions over the same metallicity range, showing thus no redshift evolution of N/O for typical galaxies over a significant fraction of cosmic time. We also show that typical $z \sim 3$ galaxies show a similar offset in the BPT diagram as galaxies from the low-z Lyman Continuum Survey (LzLCS), when compared to the average of SDSS galaxies, and show that this offset is not due to enhanced nitrogen abundances. Our results establish a basis for future studies of the evolution of N and O at higher redshifts.

Figures (5)

• Figure 1: JWST spectra from LyC22 showing object 1005 (top panel, LACES $z=3.676$ LAE in SSA22) and 300009 (bottom, $z=2.767$ LBG in the Westphal field). The blue line shows the adopted fit to the continuum. The main emission and absorption lines are indicated.
• Figure 2: Comparison of the N/O abundances derived following the direct method and the assumptions described in this work (x-axis) with those adopting the strong line calibrations of Cataldi2025Tracing-Nitroge based on JWST samples (in black; applicable for $z>1$) and an SDSS sample (red, applicable at low-$z$), and the calibration from Strom2017Nebular-Emissio (blue). All strong line calibrations use the extinction-corrected line ratio of [N ii] $\lambda$6584/[O ii] $\lambda\lambda$3727,3729.
• Figure 3: Left: Classical emission line diagnostic diagram showing the galaxies from the LyC22 sample (blue and red symbols). LyC22 objects with significant [O iii] $\lambda 4363$ detections are surrounded by a black square. LyC22 galaxies with $\log({\rm N/O})>-1.2$ are shown in red. We only show objects where the involved emission lines are detected at $\ge 3 \sigma$. Typical uncertainties are comparable to the size of the symbols, hence not plotted. The maximum starburst line from Kewley2001Optical-Classif (dotted) and the empirical AGN/star-formation threshold from Kauffmann03 established for $z \sim 0$ (dash-dotted) are also shown. Average relations for $z \sim 2.3$ galaxies from Steidel2014Strong-Nebular-Shapley2015The-MOSDEF-Surv are shown by orange and magenta lines, average relation of SDSS galaxies from Kewley2013The-Cosmic-BPT- by the red line. Observations from the LzLCS at $z\sim 0.3$ are shown by small grey squares. Right: Zoom on part of the BPT diagram showing all LyC22 sources with color-coded N/O abundances (in $\log($N/O$)$), computed from [N ii]/[O ii].
• Figure 4: Derived abundance ratio N/O from rest-optical lines of the LyC22 galaxies (blue circles) and low-$z$ samples as a function of O/H. The low-$z$ star-forming galaxies and H ii regions from the compilation of Izotov2023Abundances-of-C and the LzLCS are shown by small black and grey symbols, respectively. Dash-dotted and dotted lines show the average trend observed in low-$z$ star-forming galaxies, as parametrized by Vila-Costas1993The-nitrogen-to and Nicholls2017Abundance-scali respectively. The green dotted line shows the solar value.
• Figure 5: Excess $\Delta(\log({\rm N/O}))$ of N/O with respect to the local N/O--O/H relation from Vila-Costas1993The-nitrogen-to as a function of redshift for galaxies with abundances from the direct method and N/O from rest-optical lines. Low-$z$ star-forming galaxies and H ii regions, LzLCS and LyC22 samples are shown with the same symbols as in Fig. \ref{['fig_abund']}. Other N/O measurements from the rest-optical lines, obtained primarily from JWST spectra, are shown at $z \sim 2.2$Sanders2023A-Preview-of-JW, for the EXCELS sample Arellano-Cordova2024The-JWST-EXCELSScholte2025The-JWST-EXCELS, from Stiavelli2025What-Can-We-Lea, the Sunburst arc at $z=2.4$Welch2025The-Sunburst-Ar, and for the $z=6.1$ lensed galaxy (orange) from Topping2024Metal-poor-star, with measurements from rest-optical lines by Berg2025A-Fleeting-GLIM. The two orange symbols indicate objects known to show also UV emission lines of Nitrogen.