2025ApJ...989L..55R

Intrinsically faint galaxies at z ∼ 2-3 offer critical insights into early galaxy formation, tracing low-metallicity, low-mass systems during cosmic noon and serving as analogs to reionization-era galaxies. We present ultradeep JWST/NIRSpec spectroscopy of nine low-luminosity galaxies (-17 ≲ M_UV ≲ -20, M_⋆ ≲ 10⁹M_⊙) at z ∼ 2.5 from the CECILIA program, with ∼29.5 hr in G235M/F170LP and 1 hr in G395M/F290LP. Our sample includes four Lyα emitters, three rest-UV color-selected galaxies, and two serendipitous detections, providing the most sensitive rest-optical spectra of individual faint galaxies at this epoch to date. Balmer-line measurements reveal low star formation rates (SFRs; 0.63 < SFR/(M_⊙ yr^–1) < 5.43) and a broad range of dust reddening (0 < E(B - V) < 1), with SFRs systematically below those of continuum-selected galaxies. Electron densities are low (n_e ≲ 200 cm^–3), and emission-line diagnostics indicate low [N II]/Hα and high [O III]/Hβ, suggesting metallicities of $12+{\mathrm{log}}\,({\rm{O}}/{\rm{H}})\lesssim 8.0$. We also present the first O1-BPT constraints in such faint high-redshift galaxies. Notably, two galaxies show low [O III]/Hβ despite high Lyα EWs and very low [N II]/Hα, consistent with the predicted turnover in this ratio at very low metallicities, highlighting the need for complementary diagnostics (e.g., N2, O32) to identify metal-poor systems. Direct T_e-based abundances and expanded samples are needed to further trace metallicity and ionization trends in low-mass galaxies.