CLASSY XII: Nitrogen Enrichment Shaped by Gas Density and Feedback

TL;DR

This study examines how nitrogen-to-oxygen ratios in local and high-redshift star-forming galaxies are influenced by gas density, star formation, and feedback, revealing key factors in chemical enrichment across cosmic time.

Contribution

It provides new insights into the role of gas density and feedback in N/O enrichment, using a robust local galaxy sample and comparing with high-redshift data.

Findings

N/O ratios in local and high-redshift galaxies show no significant evolution at fixed metallicity.

Electron density correlates with N/O, indicating density structure affects chemical scatter.

Feedback processes, such as outflows, influence N/O enrichment levels.

Abstract

We investigate the chemical evolution of N/O using a sample of 45 local star-forming galaxies (SFGs) from the CLASSY survey. This sample spans a wide range of galaxy properties, with robust determinations of nitrogen and oxygen abundances via the direct-$T_{\rm e}$ method. We explore how N/O relates to density structure, stellar mass, star formation rate (SFR), stellar age, compactness, and gas kinematics. In addition, we compare our results with those of galaxies at $z =2-10$ where N/O ratios were derived from optical or UV nitrogen lines, aiming to identify chemical enrichment pathways across cosmic time. Our analysis shows that the N/O-O/H relation in CLASSY galaxies aligns with the trends seen in local galaxies and extragalactic HII regions, and that galaxies at $z = 2-6$ exhibit similar N/O values, indicating no significant redshift evolution in N/O for a fixed metallicity. We identify a significant correlation between electron density $n_{\rm e}$([S II]) and N/O, suggesting that density structure contributes to the scatter in the N/O-O/H relation. The CLASSY galaxies with high SFRs or compact star formation show elevated N/O, though no strong correlation with stellar mass is found. We also find that high-velocity outflows (v$_{out}$ > 350 km/s) and low mass-loading factors are linked to elevated N/O, indicating that feedback plays a significant role. These results highlight the importance of density, star formation, and feedback from young stellar populations in shaping N/O enrichment and provide key insights for interpreting high-$z$ galaxies observed with JWST.