CLASSY XII: nitrogen enrichment shaped by gas density and feedback
Karla Z. Arellano-Córdova, Danielle A. Berg, Matilde Mingozzi, Bethan L. James, Fiorenzo Vincenzo, Nicole H. Rogers, Evan D. Skillman, R. Amorín, Fergus Cullen, Sophia R. Flury, Valentina Abril-Melgarejo, John Chisholm, Timothy G. Heckman, Matthew Hayes, Svea Hernández, Nimisha Kumari, Chiaki Kobayashi, Claus Leitherer, Crystal L. Martin, Z Martinez, T Nanayakkara, Kaelee S. Parker, Peter Senchyna, Claudia Scarlata, Martha Stephenson, Aida Wofford, X Xu, P Zhu
Abstract
ABSTRACT We investigate the chemical evolution of N/O using a sample of 45 local star-forming galaxies 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 H ii 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 ($v_{\rm out}\gtrsim 350$ km s−1) 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 James Webb Space Telescope.