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

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### 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.

### Author and article information

###### Journal
18 December 2020
###### Article
2012.10445