CLASSY XIV: The Nitrogen Exception -- Multi-Phase Enrichment and Feedback in High-$z$ Analogs
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- 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 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 systematically 0.7 dex higher than N/H, 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.
Cite
@article{arxiv.2512.06271,
title = {CLASSY XIV: The Nitrogen Exception -- Multi-Phase Enrichment and Feedback in High-$z$ Analogs},
author = {Bethan L. James and Valentina Abril-Melgarejo and Karla Z. Arellano-Córdova and Adarsh Ranjan and Kaelee S. Parker and Danielle A. Berg and Matilde Mingozzi and Alessandra Aloisi and John Chisholm and Timothy Heckman and Alaina Henry and Svea Hernandez and Kristen B. W. McQuinn and Xinfeng Xu and Chiaki Kobayashi and The CLASSY Collaboration},
journal= {arXiv preprint arXiv:2512.06271},
year = {2025}
}
Comments
32 pages, 15 figures, 10 tables, submitted to ApJ