The KMOS3D Survey: Investigating the Origin of the Elevated Electron Densities in Star-forming Galaxies at 1 ≲ z ≲ 3

• Published in: The Astrophysical journal Vol. 909; no. 1
• Main Authors: Davies, Rebecca L, ster Schreiber, N M, Genzel, R, Shimizu, T T, Davies, R I, Schruba, A, Tacconi, L J, Übler, H, Wisnioski, E, Wuyts, S, Fossati, M, Herrera-Camus, R, Lutz, D, Mendel, J T, Naab, T, Price, S H, Renzini, A, Wilman, D, Beifiori, A, Belli, S, Burkert, A, Chan, J, Contursi, A, Fabricius, M, Lee, M M, Saglia, R P, Sternberg, A
• Format: Journal Article
• Language: English
• Published: Philadelphia IOP Publishing 01-03-2021
• Subjects: Active galactic nuclei; Astrophysics; Electron density; Galactic evolution; Galaxies; H II regions; Luminosity; Molecular clouds; Pressure; Red shift; Star & galaxy formation; Star formation; Star formation rate; Stars; Stars & galaxies; Stellar evolution
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/abd551

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) \(\simeq {n}_{{{\rm{H}}}_{2}}\), 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.