What can we learn from the Nitrogen abundance of High-z galaxies?

TL;DR

This study measures nitrogen and oxygen abundances in high-redshift galaxies using JWST spectroscopy, revealing mostly low-metallicity ratios similar to local dwarf galaxies, but some show nitrogen overabundance, suggesting complex enrichment histories.

Contribution

It provides new direct measurements of N/O ratios in galaxies at z>3 using JWST, highlighting the diversity in nitrogen enrichment at early cosmic times.

Findings

Most galaxies have N/O ratios similar to local dwarf galaxies.

Some galaxies exhibit nitrogen overabundance relative to oxygen.

Dilution by pristine gas infall is a plausible explanation for observed ratios.

Abstract

We present measurements of the gas-phase Oxygen and Nitrogen abundances obtained by applying the direct method to JWST NIRspec $R\sim1000$ spectroscopy for 6 galaxies at redshift greater than 3. Our measurements are based on rest-frame optical Nitrogen [N II]$_{\lambda\lambda6548,6583}$ lines and are complemented by 6 additional objects from the literature at $3\leq z \leq 6$. We find that 9 out of 12 objects have values of log(N/O) that are compatible with those found for low-redshift, metal-poor, dwarf galaxies and for HII regions of more luminous local galaxies. However, 3 out of 12 objects have log(N/O) values that are overabundant compared to what is expected on the basis of their Oxygen abundance. We explore a few standard scenarios to explain the observations and conclude that, within the limited statistics available to us, none of them can be definitely excluded even though we prefer dilution by pristine gas infall in between star formation bursts as this is predicted by simulations to take place as a natural part of bursty star formation.