Abstract
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]λ6716/[S ii]λ6731 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 Hα 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 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)
, 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.
Publisher
American Astronomical Society
Subject
Space and Planetary Science,Astronomy and Astrophysics
Cited by
26 articles.
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