CECILIA: Ultra-Deep Rest-Optical Spectra of Faint Galaxies at Cosmic Noon

Intrinsically faint galaxies at $z\sim 2-3$ 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 ultra-deep JWST/NIRSpec spectroscopy of nine low-luminosity galaxies ($-17\lesssim M_{\mathrm{U}\mathrm{V}}\lesssim -20$, $M_{\star }\lesssim {10}^9{M}_{\odot }$) at $z\sim 2.5$ from the CECILIA program, with $\sim$29.5 hr in G235M/F170LP and 1 hr in G395M/F290LP. Our sample includes four LAEs, 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 SFRs ($0.63<\mathrm{S}\mathrm{F}\mathrm{R}\mathrm{/}(M_{\odot }{\mathrm{y}\mathrm{r}}^{-1})<5.43$) 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 ($n_e\lesssim 200$cm${}^{-3}$), and emission-line diagnostics indicate low [NII]/H$\alpha$, high [OIII]/H$\beta$, suggesting metallicities $12+\log (\mathrm{O}\mathrm{/}\mathrm{H})\lesssim 8.0$. We also present the first O1-BPT constraints in such faint high-redshift galaxies. Notably, two galaxies show low [OIII]/H$\beta$ despite high Ly$\alpha$ EWs and very low [NII]/H$\alpha$, 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.