Gas-Phase Metallicity and Nitrogen Abundances in Low-Mass Galaxies Down to $M_{\star }\simeq 10^{5.7}{M}_{\odot }$ at $z\simeq 4.5$--$10.1$ from JWST Lensing Cluster Surveys

Abstract

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 $M_{\mathrm{UV}}\simeq -12$ to $-22$ and reaches $M_{\star }\simeq 10^{5.7}{M}_{\odot }$, with 50\% of the sources at $M_{\mathrm{UV}}>-17.5$ 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 $M_{\star }\simeq 10^{6.6}{M}_{\odot }$. Applying these calibrations, we trace the $z\sim 6$ mass--metallicity relation down to $M_{\star }\simeq 10^{6.6}{M}_{\odot }$, where it reaches $12+\log (O/H)\simeq 7.2$, with a low-mass slope slightly steeper than the local relation and in broad agreement with hydrodynamical simulations.} In the $M_{\star }\simeq 10^{7.7}{M}_{\odot }$ stack, N/O from N\,{\sc iv}]\,$\lambda \lambda 1483,1486$ exceeds that from [N\,{\sc ii}]\,$\lambda 6583$ by $\simeq 1.4$ 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 $M_{\star }\lesssim 10^6{M}_{\odot }$. \Add{The combination of these abundance patterns and a He\,{\sc ii}\,\W4686/H$\beta$ ratio of $\simeq 0.03$, 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.

35 pages, submitted to the Astrophysical Journal