CECILIA: Ultradeep Rest-optical Spectra of Faint Galaxies at Cosmic Noon

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 ⋆ ≲ 10 9 M ⊙ ) 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 Ly α 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 ]/H α and high [O III ]/H β , suggesting metallicities of 12 + log ( O / H ) ≲ 8.0 . We also present the first O1-BPT constraints in such faint high-redshift galaxies. Notably, two galaxies show low [O III ]/H β despite high Ly α EWs and very low [N II ]/H α , 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.