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Redshift Evolution of the Electron Density in the ISM at z\sim 0-9 Uncovered with JWST/NIRSpec Spectra and Line-Spread Function Determinations Published: 1/17/2023 1:13:53 PM Updated: 8/23/2023 1:00:09 PM

Paper abstract: We present electron densities n_{\rm e} in the inter-stellar medium (ISM)of star-forming galaxies at z=4-9 observed by the JWST/NIRSpec GLASS, ERO,and CEERS programs. We carefully evaluate line-spread functions of the NIRSpecinstrument as a function of wavelength with the calibration data of a planetarynebula taken onboard, and obtain secure [OII]\lambda\lambda3726,3729 doubletfluxes for 14 galaxies at z=4.02-8.68 falling on the star-formation mainsequence with the NIRSpec high and medium resolution spectra. We thus derivethe electron densities of singly-ionized oxygen nebulae with the standardn_{\rm e} indicator of [OII] doublet, and find that the electron densities ofthe z=4-9 galaxies are n_{\rm e}\gtrsim 300 cm^{-3} significantly higherthan those of low-z galaxies at a given stellar mass, star-formation rate(SFR), and specific SFR. Interestingly, typical electron densities of singlyionized nebulae increase from z=0 to z=1-3 and z=4-9, which isapproximated by the evolutionary relation of n_{\rm e}\propto(1+z)^{p} withp~ 1-2. Although it is not obvious that the ISM property of n_{\rm e} isinfluenced by global galaxy properties, these results may suggest that nebuladensities of high-z galaxies are generally high due to the compactmorphologies of high-z galaxies evolving by r_{\rm e} approximatelyproportional to (1+z)^{-1} (r_{\rm vir} \propto (1+z)^{-1}) for a givenstellar (halo) mass whose inverse square corresponds to the p~ 2evolutionary relation. The p~ 1-2 evolutionary relation can be explainedby a combination of the compact morphology and the reduction of n_{\rm e} dueto the high electron temperature of the high-z metal poor nebulae.