JWST NIRSpec High-resolution Spectroscopy of MACS0647-JD at z=10.167: Resolved [OII] Doublet and Electron Density in an Early Galaxy

We present JWST/NIRSpec high-resolution spectroscopy G395H/F290LP 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 (UV) and blue optical, including the resolved [OII]3726,3729 doublet, [NeIII]3870, [HeI]3890, Hδ, Hγ, and [OIII]4363. This is the first observation of the resolved [OII]3726,3729 doublet for a galaxy at z>8. We measure a line flux ratio [OII]3729/3726 = 0.9 ± 0.3, which corresponds to an estimated electron density of log(n_e / cm^-3) = 2.9 ± 0.5. This is significantly higher than the electron densities of local galaxies reported in the literature. We compile the 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^+31_-23 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.