The Evolution of Low-ionization Electron Densities in Galaxies across Cosmic Epochs

Abstract

Abstract We investigate the low-ionization electron density in the interstellar medium (ISM) of galaxies across 0 < z < 8, derived with the [O ii ] λλ 3727, 3730 and [S ii ] λλ 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 〉 ∼ 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 n e [S ii ] = 46 3 − 203 + 281 cm − 3 for CONGRESS, n e [S ii ] = 30 1 − 206 + 388 cm − 3 for FRESCO, and n e [O ii ] = 20 2 − 102 + 145 cm − 3 for the NIRSpec/MSA sample. The median low-ionization n e increases from z = 0 to 4, in agreement with previous studies and those using JWST observations. However, beyond z ∼ 4 median n 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 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.