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
Naveen A. Reddy, Michael W. Topping, Ryan L. Sanders, Alice E. Shapley, Gabriel Brammer
Abstract
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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mi>U</mml:mi> </mml:math> 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.