The KMOS3DSurvey: Investigating the Origin of the Elevated Electron Densities in Star-forming Galaxies at 1 ≲ z ≲ 3

Rebecca L. Davies, N. M.Förster Schreiber, R. Genzel, T. T. Shimizu, R. I. Davies, A. Schruba, L. J. Tacconi, H. Obler, E. Wisnioski, S. Wuyts, M. Fossati, R. Herrera-Camus, D. Lutz, J. T. Mendel, T. Naab, S. H. Price, A. Renzini, D. Wilman, A. Beifiori, S. BelliA. Burkert, J. Chan, A. Contursi, M. Fabricius, M. M. Lee, R. P. Saglia, A. Sternberg

Research output: Contribution to journalReview articlepeer-review


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 187-132+140 cm-3 at z ∼ 2.2 to 32-9+4 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) nH2, 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.

Original languageEnglish
Article number78
JournalAstrophysical Journal
Issue number1
StatePublished - 1 Mar 2021

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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