Stellar Chefs of the Early Universe: Where Does the Excess Nitrogen in Dwarf Galaxies Come From? ✛

Original abstract

as written by the authors arXiv:2607.15515v1

We analyze 405 deep JWST/NIRSpec spectra of star-forming galaxies at $$z=4.5$$--$$10.1$$ from DREAMS and other lensing-cluster surveys to study chemical enrichment in intrinsically faint, low-mass galaxies. The sample covers $$M_{\rm UV}\simeq-12$$ to $$-22$$ and reaches $$M_\star\simeq10^{5.7}\,M_\odot$$, with 50\% of the sources at $$M_{\rm UV}>-17.5$$ magnified by $$\mu>3$$. From individual spectra and mass-binned stacks, we derive the gas-phase metallicity together with nitrogen and carbon abundances using the rest-frame UV and optical lines. \Add{We derive new empirical strong-line metallicity calibrations using direct-method measurements from stellar-mass-binned stacks, reaching a representative stellar mass of $$M_\star\simeq10^{6.6}\,M_\odot$$. Applying these calibrations, we trace the $$z\sim6$$ mass--metallicity relation down to $$M_\star\simeq10^{6.6}\,M_\odot$$, where it reaches $$12+\log({\rm O/H})\simeq7.2$$, with a low-mass slope slightly steeper than the local relation and in broad agreement with hydrodynamical simulations.} In the $$M_\star\simeq10^{7.7}\,M_\odot$$ stack, N/O from N\,{\sc iv}]\,$$\lambda\lambda\,1483,1486$$ exceeds that from [N\,{\sc ii}]\,$$\lambda6583$$ by $$\simeq1.4$$ dex. The UV--optical difference could indicate a localized, highly ionized N-rich component whose high N/O and subsolar C/O resemble nitrogen-rich globular-cluster populations with $$M_\star\lesssim10^6\,M_\odot$$. \Add{The combination of these abundance patterns and a He\,{\sc ii}\,\W4686/H$$\beta$$ ratio of $$\simeq0.03$$, well above BPASS predictions, suggests that WR stars may contribute both prompt CNO-cycle enrichment and hard ionizing radiation, with the inferred WR population potentially supplying enough nitrogen to account for the excess on globular-cluster scales.