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We present electron densities n ₄ 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 NIRSpec instrument as a function of wavelength with the calibration data of a planetary nebula taken onboard, and obtain secure OIIλλ3726, 3729 doublet fluxes for 14 galaxies at z=4. 02-8. 68 falling on the star-formation main sequence with the NIRSpec high and medium resolution spectra. We thus derive the electron densities of singly-ionized oxygen nebulae with the standard n ₄ indicator of OII doublet, and find that the electron densities of the z=4-9 galaxies are n ₄ 300 cm^-3 significantly higher than those of low-z galaxies at a given stellar mass, star-formation rate (SFR), and specific SFR. Interestingly, typical electron densities of singly ionized nebulae increase from z=0 to z=1-3 and z=4-9, which is approximated by the evolutionary relation of n ₄ (1+z) ^p with p 1-2. Although it is not obvious that the ISM property of n ₄ is influenced by global galaxy properties, these results may suggest that nebula densities of high-z galaxies are generally high due to the compact morphologies of high-z galaxies evolving by r ₄ approximately proportional to (1+z) ^-1 (r ₕ₈ₑ (1+z) ^-1) for a given stellar (halo) mass whose inverse square corresponds to the p 2 evolutionary relation. The p 1-2 evolutionary relation can be explained by a combination of the compact morphology and the reduction of n ₄ due to the high electron temperature of the high-z metal poor nebulae.
Isobe et al. (Tue,) studied this question.