article published yes Mabel G. Stephenson author John Chisholm author A. Saldana-Lopez author Jorryt J Matthee author 7439a258-f3c0-11ec-9501-9df22fe067200000-0003-2871-127X Alaina Henry author Rohan P. Naidu author Sophia R. Flury author Taylor A. Hutchison author Steven L. Finkelstein author Natalia G. Guseva author Yuri I. Izotov author Rui Marques-Chaves author Pascal A. Oesch author Naveen A. Reddy author Daniel Schaerer author JoMa department 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. https://research-explorer.ista.ac.at/download/23200/23202/2026_AstrophysicalJour_Stephenson.pdf application/pdfno IOP Publishing2026 eng 0004-637X 1538-4357 2608.2384610.3847/1538-4357/ae9eb7 10102 M. G. Stephenson <i>et al.</i>, “The evolution of low-ionization electron densities in galaxies across cosmic epochs,” <i>The Astrophysical Journal</i>, vol. 1010, no. 2. IOP Publishing, 2026. M.G. Stephenson, J. Chisholm, A. Saldana-Lopez, J.J. Matthee, A. Henry, R.P. Naidu, S.R. Flury, T.A. Hutchison, S.L. Finkelstein, N.G. Guseva, Y.I. Izotov, R. Marques-Chaves, P.A. Oesch, N.A. Reddy, D. Schaerer, The Astrophysical Journal 1010 (2026). 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.” <i>The Astrophysical Journal</i>. IOP Publishing, 2026. <a href="https://doi.org/10.3847/1538-4357/ae9eb7">https://doi.org/10.3847/1538-4357/ae9eb7</a>. 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. <i>The Astrophysical Journal</i>. IOP Publishing. <a href="https://doi.org/10.3847/1538-4357/ae9eb7">https://doi.org/10.3847/1538-4357/ae9eb7</a> Stephenson MG, Chisholm J, Saldana-Lopez A, et al. The evolution of low-ionization electron densities in galaxies across cosmic epochs. <i>The Astrophysical Journal</i>. 2026;1010(2). doi:<a href="https://doi.org/10.3847/1538-4357/ae9eb7">10.3847/1538-4357/ae9eb7</a> Stephenson, Mabel G., et al. “The Evolution of Low-Ionization Electron Densities in Galaxies across Cosmic Epochs.” <i>The Astrophysical Journal</i>, vol. 1010, no. 2, 184, IOP Publishing, 2026, doi:<a href="https://doi.org/10.3847/1538-4357/ae9eb7">10.3847/1538-4357/ae9eb7</a>. 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. 232002026-10-27T10:40:40Z2026-10-28T08:05:42Z