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Peer-reviewedCC BY 4.0The Astrophysical Journal

The Evolution of Low-ionization Electron Densities in Galaxies across Cosmic Epochs

  • Mabel G. Stephenson1
  • John Chisholm1
  • A. Saldana-Lopez2
  • Jorryt Matthee3
  • Alaina Henry4
  • Rohan P. Naidu5
  • Sophia R. Flury6
  • Taylor A. Hutchison1,7,8
  • Steven L. Finkelstein1
  • Natalia G. Guseva9
  • Yuri I. Izotov9
  • Rui Marques-Chaves10
3 more authors
  • Pascal A. Oesch10,11
  • Naveen A. Reddy12
  • Daniel Schaerer10,13
13 affiliations
  1. 1The University of Texas at Austin
  2. 2Stockholm University
  3. 3Institute of Science and Technology Austria (ISTA)
  4. 4Space Telescope Science Institute
  5. 5Massachusetts Institute of Technology
  6. 6University of Edinburgh
  7. 7NASA Goddard Space Flight Center
  8. 8University of Maryland
  9. 9National Academy of Sciences of Ukraine
  10. 10University of Geneva
  11. 11University of Copenhagen
  12. 12University of California, Riverside
  13. 13CNRS
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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.

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Cite

BibTeXRIS
BibTeX
@article{stephenson2026evolution,
  title = {{The Evolution of Low-ionization Electron Densities in Galaxies across Cosmic Epochs}},
  author = {Mabel G. Stephenson and John Chisholm and A. Saldana-Lopez and Jorryt Matthee and Alaina Henry and Rohan P. Naidu and Sophia R. Flury and Taylor A. Hutchison and Steven L. Finkelstein and Natalia G. Guseva and Yuri I. Izotov and Rui Marques-Chaves and Pascal A. Oesch and Naveen A. Reddy and Daniel Schaerer},
  journal = {The Astrophysical Journal},
  year = {2026},
  month = oct,
  doi = {10.3847/1538-4357/ae9eb7},
  url = {https://iopscience.iop.org/article/10.3847/1538-4357/ae9eb7},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - The Evolution of Low-ionization Electron Densities in Galaxies across Cosmic Epochs
AU  - Mabel G. Stephenson
AU  - John Chisholm
AU  - A. Saldana-Lopez
AU  - Jorryt Matthee
AU  - Alaina Henry
AU  - Rohan P. Naidu
AU  - Sophia R. Flury
AU  - Taylor A. Hutchison
AU  - Steven L. Finkelstein
AU  - Natalia G. Guseva
AU  - Yuri I. Izotov
AU  - Rui Marques-Chaves
AU  - Pascal A. Oesch
AU  - Naveen A. Reddy
AU  - Daniel Schaerer
PY  - 2026
DA  - 2026/10/15
JO  - The Astrophysical Journal
DO  - 10.3847/1538-4357/ae9eb7
UR  - https://iopscience.iop.org/article/10.3847/1538-4357/ae9eb7
AB  - 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.
SN  - 0004-637X
SN  - 1538-4357
ER  -