The KMOS3D 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 KMOS3D (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]lambda 6716/[S II]lambda 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 similar to 2.2 to 32(-9)(+4) cm(-3) at z similar to 0, consistent with previous results. We use the H alpha 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 less than or similar to z less than or similar to 2.6. The KMOS3D and SAMI galaxies follow a roughly monotonic trend between n(e) and star formation rate, but the KMOS3D 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) similar or equal to n(H2), 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.