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

TL;DR

Deep JWST spectra of star-forming galaxies at z~3 reveal nitrogen-to-oxygen ratios similar to low-redshift galaxies, indicating no significant evolution in N/O over cosmic time.

Contribution

First homogeneous N/O measurements at z~3 using auroral lines, showing no redshift evolution in N/O for typical galaxies.

Findings

N/O ratios at z~3 match low-z galaxy values

No redshift evolution of N/O over a large metallicity range

Offset in BPT diagram not caused by nitrogen enhancement

Abstract

Using deep medium-resolution JWST rest-optical spectra of a sample of typical star-forming galaxies (Lyman break galaxies and Lyman-$\alpha$ 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.