The KMOS\(^{\rm 3D}\) Survey: Investigating the Origin of the Elevated Electron Densities in Star-Forming Galaxies at \(1\lesssim{z}\lesssim{3}\)

Abstract

We investigate what drives the redshift evolution of the typical electron density (\(n_e\)) in star-forming galaxies, using a sample of 140 galaxies drawn primarily from KMOS\(^{\rm 3D}\) (\(0.6\lesssim{z}\lesssim{2.6}\)) and 471 galaxies from SAMI (\(z<0.113\)). We select galaxies that do not show evidence of AGN activity or outflows, to constrain the average conditions within H II regions. Measurements of the [SII]\(\lambda\)6716/[SII]\(\lambda\)6731 ratio in four redshift bins indicate that the local \(n_e\) in the line-emitting material decreases from 187\(^{+140}_{-132}\) cm\(^{-3}\) at \(z\sim\) 2.2 to 32\(^{+4}_{-9}\) cm\(^{-3}\) at \(z\sim\) 0; consistent with previous results. We use the H\(\alpha\) luminosity to estimate the root-mean-square (rms) \(n_e\) averaged over the volumes of star-forming disks at each redshift. The local and volume-averaged \(n_e\) evolve at similar rates, hinting that the volume filling factor of the line-emitting gas may be approximately constant across \(0\lesssim{z}\lesssim{2.6}\). The KMOS\(^{\rm 3D}\) and SAMI galaxies follow a roughly monotonic trend between \(n_e\) and star formation rate, but the KMOS\(^{\rm 3D}\) galaxies have systematically higher \(n_e\) than the SAMI galaxies at fixed offset from the star-forming main sequence, suggesting a link between the \(n_e\) evolution and the evolving main sequence normalization. We quantitatively test potential drivers of the density evolution and find that \(n_e\)(rms) \(\simeq{n_{H_2}}\), suggesting that the elevated \(n_e\) in high-\(z\) H II regions could plausibly be the direct result of higher densities in the parent molecular clouds. There is also tentative evidence that \(n_e\) could be influenced by the balance between stellar feedback, which drives the expansion of H II regions, and the ambient pressure, which resists their expansion.