The evolution of low-ionization electron densities in galaxies across cosmic epochs 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. 1010 2 IOP Publishing application/pdf