On the Average Ultraviolet Emission-line Spectra of High-redshift Galaxies: Hot and Cold, Carbon-poor, Nitrogen Modest, and Oozing Ionizing Photons

Abstract We determine the spectroscopic properties of ≃1000 ostensibly star-forming galaxies at redshifts ( z = 4–10) using prism spectroscopy from JWST/NIRSpec. With rest-wavelength coverage between Ly α and [S ii ] in the optical, we stack spectra as a function of nebular conditions, and compare UV spectral properties with stellar age. This reveals UV lines of N iii ], N iv ], C iii ], C iv , He ii , and O iii ] in the average high- z galaxy. All of the UV lines are more intense in younger starbursts. We measure electron temperatures from the collisionally excited [O iii ] line ratios, finding T e = 18,000–22,000 K for the O ++ regions. We also detect a significant nebular Balmer jump, from which we estimate only T e = 8000–13,000 K. Accounting for typical temperature offsets between zones bearing doubly and singly ionized oxygen, these two temperatures remain discrepant by around 40%. We use the [O iii ] temperatures to estimate abundances of carbon, nitrogen, and oxygen. We find that log(C/O) is consistently ≃ −1, with no evolution of C/O with metallicity or stellar age. The average spectra are mildly enhanced in nitrogen, with higher N/O than low- z starbursts, but are less enhanced than samples of recently reported, high- z , extreme galaxies that show N iii ] and N iv ] emission in the UV. Whatever processes produce the N-enhancement in the individual galaxies must also be ongoing, at lower levels, in the median galaxy in the early Universe. The strongest starbursts are a source of significant ionizing emission: ionizing photon production efficiencies reach 10 25.7 Hz erg −1 , and show multiple signatures of high Lyman continuum escape, including Mg ii escape fractions nearing 100%, significant deficits in [S ii ] emission, high degrees of ionization, and blue UV colors.