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We conduct a statistical analysis of the factors responsible for thevariation in the ionization parameter (U) of high-redshift star-forminggalaxies based on medium resolution JWST/NIRSpec observations obtained by theCosmic Evolution Early Release Science (CEERS) survey. The sample consists of48 galaxies with spectroscopic redshifts z=2.7-6.3 which are largelyrepresentative of typical star-forming galaxies at these redshifts. The [SII]6718, 6733 doublet is used to estimate electron densities (n_e), anddust-corrected Ha luminosities are used to compute total ionizing photon rates(Q). Using composite spectra of galaxies in bins of [OIII]/[OII] (i.e., O32) asa proxy for U, we determine that galaxies with higher O32 have <n_e> ~ 500cm^-3 that are at least a factor of ~5 larger than that of lower-O32 galaxies.We do not find a significant difference in <Q> between low- and high-O32galaxies. Photoionization modeling of all available strong rest-frame opticalemission lines is used to simultaneously constrain U and oxygen abundance(Z_neb). We find a large spread in log U of ~1.5 dex at a fixed Z_neb. On theother hand, the data indicate a highly significant correlation between U andstar-formation-rate surface density (Sigma_SFR) which appears to be redshiftinvariant at z~1.6-6.3, and possibly up to z~9.5. We consider several avenuesthrough which metallicity and Sigma_SFR (or gas density) may influence U,including variations in n_e and Q that are tied to metallicity and gas density,internal dust extinction of ionizing photons, and the effects of gas density onthe volume filling fraction of dense clumps in HII regions and the escapefraction of ionizing photons. Based on these considerations, we conclude thatgas density may play a more central role than metallicity in modulating U atthese redshifts.

astrophysical journal/the astrophysical journal

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