On the Average Ultraviolet Emission Line Spectra of High-Redshift Galaxies: Hot and Cold, Carbon-poor, Nitrogen-modest, and Oozing Ionizing Photons

TL;DR

This study analyzes the UV emission spectra of around 1000 high-redshift galaxies using JWST data, revealing insights into their nebular conditions, chemical abundances, and ionizing photon production, with implications for early galaxy evolution.

Contribution

It provides the first large-scale spectroscopic analysis of high-z galaxies' UV lines, measuring temperatures, abundances, and ionizing efficiencies, highlighting ongoing nitrogen enrichment and ionizing photon escape.

Findings

UV lines are more intense in younger starbursts.

Electron temperatures range from 8000 to 22000 K.

High ionizing photon production efficiencies observed.

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 Lya 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 UV lines are more intense in younger starbursts. We measure electron temperatures from the collisionally excited [O III] line ratios, finding Te=18000-22000 K for the O++ regions. We also detect a significant nebular Balmer Jump from which we estimate only Te=8000-13000 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 high-z galaxies with visible UV N III] and N IV]. 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, 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.