A JWST/NIRSpec Exploration of the Connection between Ionization Parameter, Electron Density, and Star-formation-rate Surface Density in z = 2.7–6.3 Galaxies

Abstract

We examine the factors responsible for the variation in the ionization parameter ( U ) of high-redshift star-forming galaxies based on medium-resolution JWST/NIRSpec observations obtained by the Cosmic Evolution Early Release Science survey. The sample consists of 48 galaxies with redshifts z _spec = 2.7−6.3, which are largely representative of typical galaxies at these redshifts. The [S ii ] λ λ 6718, 6733 doublet is used to estimate electron densities ( n _e ), and dust-corrected H α luminosities are used to compute ionizing photon rates ( Q ). Using composite spectra of galaxies in bins of [O iii ] λ λ 4960, 5008/[O ii ] λ λ 3727, 3730 (O32) as a proxy for U , we determine that galaxies with higher O32 have 〈 n _e 〉 ≃ 500 cm ^−3 that are ≳5 × larger than that of lower-O32 galaxies. We do not find a significant difference in 〈 Q 〉 between low- and high-O32 galaxies. Photoionization modeling indicates a large spread in $\mathrm{log}U$ of ≈1.5 dex at a fixed Z _neb . On the other hand, the data indicate a highly significant correlation between U and star-formation-rate surface density (Σ _SFR ), which appears to be redshift invariant at z ∼ 1.6−6.3, and possibly up to z ∼ 9.5. We consider several avenues through which metallicity and Σ _SFR (or gas density) may influence U , including variations in n _e and Q , internal dust extinction of ionizing photons, and the effects of gas density on the volume filling fraction. Based on these considerations, we conclude that gas density may play a more central role than metallicity in modulating U at these redshifts.

Keywords

• High-redshift galaxies
• Starburst galaxies
• Interstellar abundances
• Stellar abundances
• H II regions
• Interstellar medium