Potential Nitrogen Enrichment via Direct-Collapse Wolf-Rayet Stars in a $z=4.7$ Star-Forming Galaxy

Abstract

We present analyses of a nitrogen-enriched star-forming galaxy, ID60001, at $z=4.6928$ based on JWST/NIRSpec MSA spectroscopy and NIRCam photometry. From rest-frame optical emission lines we derive the nitrogen-to-oxygen (N/O) abundance ratio of ID60001 to be $\log({\rm N/O})=-0.76_{-0.03}^{+0.03}$ ($[{\rm N/O}]=0.10_{-0.03}^{+0.03}$), which is significantly elevated at the corresponding metallicity $12+\log({\rm O/H})=7.75_{-0.01}^{+0.01}$ ($Z/Z_\odot = 0.12$) compared to local counterparts. We discuss possible scenarios for elevated N/O abundance in ID60001, including pristine gas inflow, Wolf-Rayet (WR) stars, and Oxygen depletion by Type II supernova winds. Based on the moderately broadened He{\sc ii}$\lambda$4686 emission line, galaxy morphology, and star-formation history, we conclude that the elevated N/O abundance of ID60001 is likely originated from massive ($>25\,M_\odot$) WR stars that directly collapse into a black hole. We also stress the importance of reliable electron density measurements when deriving N/O abundance with rest-frame optical emission lines.