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 ∼ 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. Our sample includes Ly α -selected and other low-luminosity star-forming galaxies with low stellar masses log ( M * / M ⊙ ) ≈ 7.3 –9.4 and moderately faint rest-UV magnitudes (−20.7 ≲ M UV ≲ −17.3). Gas-phase oxygen abundances, calculated using empirical calibrations of [O iii ]/H β together with [N ii ]/H α constraints, span ∼0.04–0.5 Z ⊙ . We measure a steep MZR slope of γ = 0.52 ± 0.13, 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 ∼ 0 to z ∼ 2, reflecting less metal enrichment in early galaxies. We find no significant evolution in the MZR between z ∼ 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 buildup 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.