CLASSY XIV: The Nitrogen Exception -- Multi-Phase Enrichment and Feedback in High-$z$ Analogs

TL;DR

By comparing N, O, S, and Fe abundances in neutral and ionized gas across 31 local star-forming galaxies from CLASSY, the study probes how metals cycle through the ISM on short and long timescales. O and S show near-uniform abundances between phases with scatter from local inhomogeneity, while Fe is enhanced in the neutral gas, suggesting delayed mixing or dust depletion in the ionized phase. Nitrogen shows the largest phase offset, with N/H$_{ion}$ roughly $0.7$ dex higher than N/H$_{neutral}$, and N/O in the ionized gas rises within $3-6$ Myr, consistent with Wolf-Rayet enrichment rather than late-time AGB production. The offset correlates with stellar mass, metallicity, SFR, and most strongly with ISM outflow velocity, implying that localized stellar feedback and phase-dependent mixing regulate chemical evolution and help explain extreme N/O seen in early cosmic epochs.

Abstract

We present a first-of-its-kind analysis of the metal content across two interstellar medium (ISM) phases in a sample of 31 local star-forming galaxies from the COS Legacy Archive Spectroscopic SurveY (CLASSY), selected as analogues of high-$z$ systems. Using co-spatial UV absorption and optical emission-line spectroscopy, we compare abundances of N, O, S, and Fe in the low-ionization (neutral) and high-ionization (ionized) gas, providing a multi-phase view of enrichment shortly after the current starburst and over longer timescales when ejecta from previous episodes have cooled and mixed. We find that O and S, produced predominantly in short-lived massive stars, are well mixed between the two phases, with scatter reflecting local inhomogeneities. Fe, predominantly produced by Type Ia supernovae on $\sim$1 Gyr timescales, is higher in the neutral gas, reflecting either delayed mixing of older Fe-enriched material or preferential depletion of Fe from the ionized phase through dust formation in core-collapse supernova ejecta. N exhibits the largest phase offset, with N/H$_{ion}$ systematically $\sim$0.7 dex higher than N/H$_{neu}$, and the magnitude of this offset correlates with stellar mass, metallicity, star-formation rate, and most strongly with the ISM outflow velocity. N/O ratios in the ionized phase rise rapidly within 3-6 Myr relative to the neutral gas, consistent with N enrichment dominated by Wolf-Rayet stars rather than intermediate-mass AGB stars on longer timescales. These results demonstrate that localized stellar feedback, outflows, and phase-dependent mixing collectively regulate the chemical evolution of star-forming galaxies, providing key insight into the extreme N/O abundances recently observed in galaxies at cosmic dawn.