CECILIA: The Mass-Metallicity Relation of Low-Mass Galaxies at Cosmic Noon

Abstract

A galaxy's metallicity and its relation to stellar mass encode the history of gas accretion, star formation, and outflows within cosmic ecosystems. We present new constraints on the low-mass end of the mass-metallicity relation (MZR) at $z\sim 2-3$ from ultra-deep JWST/NIRSpec spectroscopy of seven continuum-faint galaxies in the Chemical Evolution Constrained using Ionized Lines in Interstellar Aurorae (CECILIA) Faint sample (Raptis et al. 2025). Our sample includes Ly$\hat{I}\pm$-selected and other low-luminosity star-forming galaxies with stellar masses $\log (M_{\star }/M_{\odot })\sim 7.2-9.7$ and moderately faint rest-UV magnitudes ($-20.7\lesssim M_{\mathrm{UV}}\lesssim -17.3$). Gas-phase oxygen abundances, calculated using empirical calibrations of [O III]/H${\hat{I}}^2$ together with [N II]/H$\hat{I}\pm$ constraints, span $\sim 0.04-0.5$ $Z_{\odot }$. We measure a steep MZR slope of ${\hat{I}}^3=0.48\pm 0.11$, suggesting a rapid increase in metal retention efficiency with mass, consistent with energy-driven outflows. Comparison with lower- and higher-redshift studies indicates an evolution in normalization from $z\sim 0$ to $z\sim 2$, reflecting less metal enrichment in early galaxies. We find no significant evolution in the MZR between $z\sim 2$ and the Epoch of Reionization, suggesting that our galaxies may serve as useful analogs of reionization-era systems. Expanded samples and direct $T_e$-based abundance measurements will be crucial to fully trace the build-up of metals in low-mass galaxies during the peak epoch of cosmic star formation and to test the reliability of strong-line calibrations in these galaxies.

Submitted to The Astrophysical Journal Letters. 18 pages, 7 figures, 2 tables