The KMOS${}^{3D}$ 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${}^{3D}$ ($0.6\lesssim z\lesssim 2.6$) 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]$\hat{I}»$6716/[SII]$\hat{I}»$6731 ratio in four redshift bins indicate that the local $n_e$ in the line-emitting material decreases from 187${}_{-132}^{+140}$ cm${}^{-3}$ at $z\sim$ 2.2 to 32${}_{-9}^{+4}$ cm${}^{-3}$ at $z\sim$ 0; consistent with previous results. We use the H$\hat{I}\pm$ 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${}^{3D}$ and SAMI galaxies follow a roughly monotonic trend between $n_e$ and star formation rate, but the KMOS${}^{3D}$ 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.

Main text 24 pages, 8 figures. Accepted for publication in ApJ