2024ApJ...973...47A

We present JWST/NIRSpec high-resolution G395H/F290LP spectroscopy of MACS0647-JD, a gravitationally lensed galaxy merger at z = 10.167. The new spectroscopy, which is acquired for the two lensed images (JD1 and JD2), detects and resolves emission lines in the rest-frame ultraviolet and blue optical, including the resolved [O II] λλ3726, 3729 doublet, [Ne III] λ3870, He Iλ3890, Hδ, Hγ, and [O III] λ4363. This is the first observation of the resolved [O II] λλ3726, 3729 doublet for a galaxy at z > 8. We measure a line flux ratio [O II] λ3729/λ3726 = 0.9 ± 0.3, which corresponds to an estimated electron density of $\mathrm{log}({n}_{e}/{\mathrm{cm}}^{-3})=2.9\pm 0.5$. This is significantly higher than the electron densities of local galaxies reported in the literature. We compile measurements from the literature and further analyze the redshift evolution of n_e. We find that the redshift evolution follows the power-law form of n_e = A × (1 + z)^p^ with $A={54}_{-23}^{+31}$ cm^–3 and $p={1.2}_{-0.4}^{+0.4}$. This power-law form may be explained by a combination of metallicity and morphological evolution of galaxies, which become, on average, more metal poor and more compact with increasing redshift.