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

Abstract

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, H$\hat{I}´$, H${\hat{I}}^3$, 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 $=0.9\pm 0.3$, which corresponds to an estimated electron density of $\log (n_e/{\mathrm{cm}}^{-3})=2.9\pm 0.5$. This is significantly higher than the electron densities of local galaxies reported in the literature. We compile 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 $n_e=A\times (1+z{)}^p$ 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.

20 pages, 11 figures. Accepted for publication in ApJ