The KMOS 3D Survey: Investigating the Origin of the Elevated Electron Densities in Star-forming Galaxies at 1 ≲ z ≲ 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 < z < 2.6) and 471 galaxies from SAMI ( z < 0.113). We select galaxies that do not show evidence of active galactic nucleus activity or outflows to constrain the average conditions within H ii regions. Measurements of the [S ii ] λ 6716/[S ii ] λ 6731 ratio in four redshift bins indicate that the local n e in the line-emitting material decreases from 187 cm −3 at z ∼ 2.2 to 32 cm −3 at z ∼ 0, consistent with previous results. We use the H α luminosity to estimate the 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 ≲ z ≲ 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 a 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) , 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.