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

Abstract

We investigate the low-ionization electron density in the interstellar medium of galaxies across $0$$<$$z$$<$$8$, derived with the [O II]$\lambda \lambda 3727,3730$ and [S II]$\lambda \lambda 6718,6733$ 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 $⟨z⟩\sim 5.6$; 691 galaxies in the luminosity-complete NIRCam/grism programs CONGRESS and FRESCO at $⟨z⟩\sim 4.3$ and $⟨z⟩\sim 5.3$, respectively, and 12 galaxies with high-resolution NIRSpec/MSA spectra from the GO 1871 and GLASS programs. We find median electron densities of $n_{\mathrm{e}}$[S II]$=$$463_{-203}^{+281}{\mathrm{cm}}^{-3}$ for CONGRESS, $n_{\mathrm{e}}$[S II]$=$$301_{-206}^{+388}{\mathrm{cm}}^{-3}$ for FRESCO, and $n_{\mathrm{e}}$[O II]$=$$202_{-102}^{+145}{\mathrm{cm}}^{-3}$ for the NIRSpec/MSA sample. The median low-ionization $n_{\mathrm{e}}$ increases from $z=0$ to $z=4$, in agreement with previous studies and those using JWST observations. However, beyond $z\sim 4$ median $n_{\mathrm{e}}$ 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 $n_{\mathrm{e}}$, 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.