The evolution of low-ionization electron densities in galaxies across cosmic epochs
Stephenson MG, Chisholm J, Saldana-Lopez A, Matthee JJ, Henry A, Naidu RP, Flury SR, Hutchison TA, Finkelstein SL, Guseva NG, Izotov YI, Marques-Chaves R, Oesch PA, Reddy NA, Schaerer D. 2026. The evolution of low-ionization electron densities in galaxies across cosmic epochs. The Astrophysical Journal. 1010(2), 184.
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Author
Stephenson, Mabel G.;
Chisholm, John;
Saldana-Lopez, A.;
Matthee, Jorryt JISTA 
;
Henry, Alaina;
Naidu, Rohan P.;
Flury, Sophia R.;
Hutchison, Taylor A.;
Finkelstein, Steven L.;
Guseva, Natalia G.;
Izotov, Yuri I.;
Marques-Chaves, Rui
All
;
Henry, Alaina;
Naidu, Rohan P.;
Flury, Sophia R.;
Hutchison, Taylor A.;
Finkelstein, Steven L.;
Guseva, Natalia G.;
Izotov, Yuri I.;
Marques-Chaves, Rui
All
Department
Abstract
We investigate the low-ionization electron density in the interstellar medium (ISM) of galaxies across 0 < z < 8,
derived with the [O II] λλ3727, 3730 and [S II] λλ6718, 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 ne[S II] = + 463 203 cm 281 3
for CONGRESS, ne[S II] = + 301 206 cm 388 3
for FRESCO, and
ne[O II] = + 202 102 cm 145 3
for the NIRSpec/MSA sample. The median low-ionization ne increases from z = 0 to 4,
in agreement with previous studies and those using JWST observations. However, beyond z ∼ 4 median ne begins
to deviate from a monotonically increasing evolution with z. We find a positive, but shallow, correlation between
O32 (a proxy for the ionization parameter) and ne, while also confirming a positive correlation between O32 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.
Publishing Year
Date Published
2026-10-15
Journal Title
The Astrophysical Journal
Publisher
IOP Publishing
Acknowledgement
We are grateful for the support for programs GO 1871, GO 1895, and GO 3577 provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127. This work is based in part on observations made with the NASA/ESA/CSA JWST. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with programs GO 1324, GO 1871, GO 1895 and GO 3577. The specific observations analyzed can be accessed via DOI: 10.17909/01dt-fm17.
A.S.L. acknowledges support from Knut and Alice Wallenberg Foundation. T.A.H. acknowledges support from an appointment to the NASA Postdoctoral Program at the NASA Goddard Space, administered by Oak Ridge Associated Universities under contract with NASA, as well as the University of Maryland Baltimore County and the Center for Space Sciences and Technology. N.G.G. and Y.I.I. and acknowledge support from the joint Ukrainian-Swiss project No. 224866. This work has received funding from the Swiss State Secretariat for Education, Research, and Innovation (SERI) under contract number MB22.00072, as well as from the Swiss National Science Foundation (SNSF) through project grant 200020_207349. The Cosmic Dawn Center (DAWN) is funded by the Danish National Research Foundation under grant DNRF140.
Facility: JWST (NIRCam - , NIRSpec).
Software: numpy (v.1.24.3; C. R. Harris et al. 2020), scipy (v.1.13.1; P. Virtanen et al. 2020), astropy (Astropy Collaboration et al. 2013, 2018, 2022), pyneb (v.1.1.24; V. Luridiana et al. 2013, 2015; C. Morisset et al. 2020), msaexp (v.0.8.4; G. B. Brammer 2022), msafit (A. de Graaff et al. 2024), grizli (G. Brammer 2018), EAZY (G. B. Brammer et al. 2008), BAGPIPES (A. C. Carnall et al. 2018), Prospector (B. D. Johnson et al. 2019, 2021), pysersic (I. Pasha & T. B. Miller 2023), GALFIT (C. Y. Peng et al. 2002, 2010), kendall (S. R. Flury et al. 2022; S. R. Flury 2023).
Volume
1010
Issue
2
Article Number
184
ISSN
eISSN
IST-REx-ID
Cite this
Stephenson MG, Chisholm J, Saldana-Lopez A, et al. The evolution of low-ionization electron densities in galaxies across cosmic epochs. The Astrophysical Journal. 2026;1010(2). doi:10.3847/1538-4357/ae9eb7
Stephenson, M. G., Chisholm, J., Saldana-Lopez, A., Matthee, J. J., Henry, A., Naidu, R. P., … Schaerer, D. (2026). The evolution of low-ionization electron densities in galaxies across cosmic epochs. The Astrophysical Journal. IOP Publishing. https://doi.org/10.3847/1538-4357/ae9eb7
Stephenson, Mabel G., John Chisholm, A. Saldana-Lopez, Jorryt J Matthee, Alaina Henry, Rohan P. Naidu, Sophia R. Flury, et al. “The Evolution of Low-Ionization Electron Densities in Galaxies across Cosmic Epochs.” The Astrophysical Journal. IOP Publishing, 2026. https://doi.org/10.3847/1538-4357/ae9eb7.
M. G. Stephenson et al., “The evolution of low-ionization electron densities in galaxies across cosmic epochs,” The Astrophysical Journal, vol. 1010, no. 2. IOP Publishing, 2026.
Stephenson MG, Chisholm J, Saldana-Lopez A, Matthee JJ, Henry A, Naidu RP, Flury SR, Hutchison TA, Finkelstein SL, Guseva NG, Izotov YI, Marques-Chaves R, Oesch PA, Reddy NA, Schaerer D. 2026. The evolution of low-ionization electron densities in galaxies across cosmic epochs. The Astrophysical Journal. 1010(2), 184.
Stephenson, Mabel G., et al. “The Evolution of Low-Ionization Electron Densities in Galaxies across Cosmic Epochs.” The Astrophysical Journal, vol. 1010, no. 2, 184, IOP Publishing, 2026, doi:10.3847/1538-4357/ae9eb7.
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