The Impact of Star-formation-rate Surface Density on the Electron Density and Ionization Parameter of High-redshift Galaxies*
Naveen A. Reddy, Ryan L. Sanders, Alice E. Shapley, Michael W. Topping, Mariska Kriek, Alison L. Coil, Bahram Mobasher, Brian Siana, Saeed Rezaee
Abstract
Abstract We use the large spectroscopic data set of the MOSFIRE Deep Evolution Field 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 (Σ SFR ). Using a sample of 317 galaxies with spectroscopic redshifts z spec ≃ 1.9–3.7, we construct composite rest-frame optical spectra in bins of Σ SFR and infer electron densities, n e , using the ratio of the [O ii ] λ λ 3727, 3730 doublet. Our analysis suggests a significant (≃3 σ ) correlation between n e and Σ SFR . We further find significant correlations between U and Σ 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 H ii regions—with Σ SFR appear to be important factors in explaining the relationship between U and Σ 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.