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arXiv · 2608.23846

The Evolution of Low-Ionization Electron Densities in Galaxies Across Cosmic Epochs

Mabel G. Stephenson·John Chisholm·Alberto Saldana-Lopez·Jorryt Matthee·Alaina Henry·Rohan P. Naidu·Sophia R. Flury·Taylor A. Hutchison·Steven L. Finkelstein·Natalia G. Guseva·Yuri I. Izotov·Rui Marques-Chaves·Pascal A. Oesch·Naveen A. Reddy·Daniel Schaerer
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Abstract

We investigate the low-ionization electron density in the interstellar medium of galaxies across $0$$<$$z$$<$$8$, derived with the [O II]$\lambda\lambda3727,3730$ and [S II]$\lambda\lambda6718,6733$ 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 $\langle z\rangle\sim5.6$; 691 galaxies in the luminosity-complete NIRCam/grism programs CONGRESS and FRESCO at $\langle z\rangle \sim4.3$ and $\langle z\rangle\sim5.3$, respectively, and 12 galaxies with high-resolution NIRSpec/MSA spectra from the GO 1871 and GLASS programs. We find median electron densities of $n_{\rm{e}}$[S II]$=$$463^{+281}_{-203}~{\rm{cm}}^{-3}$ for CONGRESS, $n_{\rm{e}}$[S II]$=$$301^{+388}_{-206}~{\rm{cm}}^{-3}$ for FRESCO, and $n_{\rm{e}}$[O II]$=$$202^{+145}_{-102}~{\rm{cm}}^{-3}$ for the NIRSpec/MSA sample. The median low-ionization $n_{\rm{e}}$ increases from $z= 0$ to $z=4$, in agreement with previous studies and those using JWST observations. However, beyond $z\sim4$ median $n_{\rm{e}}$ 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 $n_{\rm{e}}$, 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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Mabel G. Stephenson, John Chisholm, Alberto Saldana-Lopez, Jorryt Matthee, Alaina Henry, Rohan P. Naidu, Sophia R. Flury, Taylor A. Hutchison, Steven L. Finkelstein, Natalia G. Guseva, Yuri I. Izotov, Rui Marques-Chaves, Pascal A. Oesch, Naveen A. Reddy, Daniel Schaerer. 2026-08-24. The Evolution of Low-Ionization Electron Densities in Galaxies Across Cosmic Epochs. https://arxiv.org/abs/2608.23846

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