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 OII3726, 3729 doublet, NeIII3870, HeI3890, H, H, and OIII4363. This is the first observation of the resolved OII3726, 3729 doublet for a galaxy at z>8. We measure a line flux ratio OII3729/3726 = 0. 9 0. 3, which corresponds to an estimated electron density of (n₄ / 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 n₄. We find that the redshift evolution follows the power-law form of n₄ = 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.