2026/07/17 by Hiroya Umeda, Masami Ouchi, Kimihiko Nakajima +7
#astro-ph.GA
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≃-12 to -22 and reaches M_⋆≃105.7 M_\odot, with 50% of the sources at M\rm UV>-17.5 magnified by μ>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. \AddWe derive new empirical strong-line metallicity calibrations using direct-method measurements from stellar-mass-binned stacks, reaching a representative stellar mass of M_⋆≃106.6 M_\odot. Applying these calibrations, we trace the z∼6 mass--metallicity relation down to M_⋆≃106.6 M_\odot, where it reaches 12+log(\rm O/H)≃7.2, with a low-mass slope slightly steeper than the local relation and in broad agreement with hydrodynamical simulations. In the M_⋆≃107.7 M_\odot stack, N/O from N \sc iv] λλ 1483,1486 exceeds that from [N \sc ii] λ6583 by ≃1.4 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_⋆\lesssim106 M_\odot. \AddThe combination of these abundance patterns and a He \sc ii \W4686/Hβ ratio of ≃0.03, 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.