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

• Published in: The Astrophysical Journal
• Main Authors: Davies, Rebecca L., Schreiber, N. M. Förster, 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.
• Other Authors: Swinburne University of Technology
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: American Astronomical Society 2021
• Subjects: sami
• Online Access: http://hdl.handle.net/1959.3/460214; https://doi.org/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) 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.