2023ApJ...949L..25F

We present JWST NIRSpec spectroscopy for 11 galaxy candidates with photometric redshifts of z ≃ 9 - 13 and M_UV ∈ [ -21, -18] newly identified in NIRCam images in the Cosmic Evolution Early Release Science Survey. We confirm emission line redshifts for 7 galaxies at z = 7.762-8.998 using spectra at ∼1-5 μm either with the NIRSpec prism or its three medium-resolution (R ∼ 1000) gratings. For z ≃ 9 photometric candidates, we achieve a high confirmation rate of ≃90%, which validates the classical dropout selection from NIRCam photometry. No robust emission lines are identified in three galaxy candidates at z > 10, where the strong [O III] and Hβ lines would be redshifted beyond the wavelength range observed by NIRSpec, and the Lyα continuum break is not detected with the sensitivity of the current data. Compared with Hubble Space Telescope-selected bright galaxies (M_UV ≃ -22) that are similarly spectroscopically confirmed at z ≃ 8 - 9, these NIRCam-selected galaxies are characterized by lower star formation rates (SFRs; SFR ≃ 4 M_⊙ yr^–1) and lower stellar masses (≃10⁸M_⊙), but with higher specific SFR (≃40 Gyr^–1), higher [O III]+Hβ equivalent widths (≃1100 Å), and elevated production efficiency of ionizing photons ($\mathrm{log}({\xi }_{\mathrm{ion}}/\mathrm{Hz}\,{\mathrm{erg}}^{-1})\simeq 25.8$) induced by young stellar populations (<10 Myr) accounting for ≃20% of the galaxy mass, highlighting the key contribution of faint galaxies to cosmic reionization. Taking advantage of the homogeneous selection and sensitivity, we also investigate metallicity and ISM conditions with empirical calibrations using the [O III]₅₀₀₈/Hβ ratio. We find that galaxies at z ≃ 8 - 9 have higher SFRs and lower metallicities than galaxies at similar stellar masses at z ≃ 2 - 6, which is generally consistent with the current galaxy formation and evolution models.