The Impact of Star-Formation-Rate Surface Density on the Electron Density and Ionization Parameter of High-Redshift Galaxies

TL;DR

This study investigates how star-formation-rate surface density influences electron density and ionization parameters in high-redshift galaxies, revealing significant correlations that explain their elevated ionization levels.

Contribution

It demonstrates that gas density and star-formation activity are key factors in the high ionization parameters of early galaxies, challenging metallicity-focused explanations.

Findings

Significant correlation between electron density and star-formation-rate surface density.

Ionization parameter correlates with star-formation-rate surface density in composite and individual galaxy spectra.

Gas density and star-formation activity largely explain the redshift evolution of ionization parameters.

Abstract

We use the large spectroscopic dataset of the MOSFIRE Deep Evolution Field (MOSDEF) survey to investigate some of the key factors responsible for the elevated ionization parameters (U) inferred for high-redshift galaxies, focusing in particular on the role of star-formation-rate surface density (Sigma_SFR). Using a sample of 317 galaxies with spectroscopic redshifts z~1.9-3.7, we construct composite rest-frame optical spectra in bins of Sigma_SFR and infer electron densities, n_e, using the ratio of the [OII] 3727, 3730 doublet. Our analysis suggests a significant (~3 sigma) correlation between n_e and Sigma_SFR. We further find significant correlations between U and Sigma_SFR for composite spectra of a subsample of 113 galaxies, and for a smaller sample of 25 individual galaxies with inferences of U. The increase in n_e -- and possibly also the volume filling factor of dense clumps in HII regions -- with Sigma_SFR appear to be important factors in explaining the relationship between U and Sigma_SFR. Further, the increase in n_e and SFR with redshift at a fixed stellar mass can account for most of the redshift evolution of U. These results suggest that the gas density, which sets n_e and the overall level of star-formation activity, may play a more important role than metallicity evolution in explaining the elevated ionization parameters of high-redshift galaxies.