The Evolution of Low-ionization Electron Densities in Galaxies across Cosmic Epochs
Abstract We investigate the low-ionization electron density in the interstellar medium (ISM) of galaxies across 0 < z < 8, derived with the [O ii ] λλ 3727, 3730 and [S ii ] λλ 6718, 6733 doublets, using both deep JWST/NIRCam/grism and NIRSpec/MSA spectroscopy. Along with ancillary density samples from the literature at 0 < z < 6, we assemble three samples containing a total of 703 galaxies spanning 3.7 < z < 7.8 at 〈 z 〉 ∼ 5.6; 691 galaxies in the luminosity-complete NIRCam/grism programs CONGRESS and FRESCO at 〈 z 〉 ∼ 4.3 and ∼5.3, respectively; and 12 galaxies with high-resolution NIRSpec/MSA spectra from the GO 1871 and GLASS programs. We find median electron densities of n e [S ii ] = 46 3 − 203 + 281 cm − 3 for CONGRESS, n e [S ii ] = 30 1 − 206 + 388 cm − 3 for FRESCO, and n e [O ii ] = 20 2 − 102 + 145 cm − 3 for the NIRSpec/MSA sample. The median low-ionization n e increases from z = 0 to 4, in agreement with previous studies and those using JWST observations. However, beyond z ∼ 4 median n e begins to deviate from a monotonically increasing evolution with z . We find a positive, but shallow, correlation between O 32 (a proxy for the ionization parameter) and n e , while also confirming a positive correlation between O 32 and z . We suggest that these trends are possibly due to an evolving ISM gas density and ionization structure, and propose a toy model where the gas clumping evolves with z to explain the observed density and ionization variations.
Authors
- Mabel G. Stephenson (ORCID: https://orcid.org/0000-0003-4717-0376)
- R. Marques-Chaves (ORCID: https://orcid.org/0000-0001-8442-1846)
- Jorryt Matthee (ORCID: https://orcid.org/0000-0003-2871-127X)
- Sophia R. Flury (ORCID: https://orcid.org/0000-0002-0159-2613)
- N. G. Guseva (ORCID: https://orcid.org/0000-0002-7938-614X)
- Alaina Henry (ORCID: https://orcid.org/0000-0002-6586-4446)
- Rohan P. Naidu (ORCID: https://orcid.org/0000-0003-3997-5705)
- Naveen A. Reddy (ORCID: https://orcid.org/0000-0001-9687-4973)
- Alberto Saldana-Lopez (ORCID: https://orcid.org/0000-0001-8419-3062)
- Y. I. Izotov (ORCID: https://orcid.org/0000-0002-1416-6082)
- Steven L. Finkelstein (ORCID: https://orcid.org/0000-0001-8519-1130)
- Pascal A. Oesch (ORCID: https://orcid.org/0000-0001-5851-6649)
- Daniel Schaerer (ORCID: https://orcid.org/0000-0001-7144-7182)
- Taylor A. Hutchison (ORCID: https://orcid.org/0000-0001-6251-4988)
- 0000-0003-4717-0376 (ORCID: https://orcid.org/0000-0002-0302-2577)
Institutions
- University of California, Riverside (US)
- University of Geneva (CH)
- University of Copenhagen (DK)
- Centre National de la Recherche Scientifique (FR)
- Space Telescope Science Institute (US)
- Goddard Space Flight Center (US)
- National Academy of Sciences of Ukraine (UA)
- Institute of Science and Technology Austria (AT)
- Stockholm University (SE)
- Massachusetts Institute of Technology (US)
- University of Maryland, College Park (US)
- The University of Texas at Austin (US)
- University of Edinburgh (GB)
Publication Details
- Journal
- The Astrophysical Journal
- Published
- 2026-10-15
- DOI
- https://doi.org/10.3847/1538-4357/ae9eb7
- Primary Topic
- Galaxies: Formation, Evolution, Phenomena
- Type
- article
- Field-Weighted Citation Impact
- 0.00
Funders
- National Science Foundation
- National Aeronautics and Space Administration
- Oak Ridge Associated Universities
- Space Telescope Science Institute
- National Research Foundation
- Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung
- Danmarks Grundforskningsfond
- Knut och Alice Wallenbergs Stiftelse
- Goddard Space Flight Center
- Staatssekretariat für Bildung, Forschung und Innovation