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

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 Lyα 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.