A Preview of JWST Metallicity Studies at Cosmic Noon: The First Detection of Auroral [O II] Emission at High Redshift

TL;DR

This study demonstrates the first detection of auroral [O II] emission at high redshift, enabling direct metallicity measurements in distant galaxies and highlighting JWST's potential for advancing early universe chemical abundance research.

Contribution

It presents the first high-redshift detection of auroral [O II] lines, allowing for direct metallicity measurements and showcasing JWST's capabilities for future studies.

Findings

Detected auroral [O II] lines at z=2.18, enabling direct metallicity measurements.

Found N/O and alpha/Fe ratios consistent with rapid star formation.

Highlighted JWST's increased efficiency for high-redshift metallicity surveys.

Abstract

We present ultra-deep Keck/MOSFIRE rest-optical spectra of two star-forming galaxies at z=2.18 in the COSMOS field with bright emission lines, representing more than 20~hours of total integration. The fidelity of these spectra enabled the detection of more than 20 unique emission lines for each galaxy, including the first detection of the auroral [O II]$\lambda\lambda$7322,7332 lines at high redshift. We use these measurements to calculate the electron temperature in the low-ionization O$^+$ zone of the ionized ISM and derive abundance ratios of O/H, N/H, and N/O using the direct method. The N/O and $\alpha$/Fe abundance patterns of these galaxies are consistent with rapid formation timescales and ongoing strong starbursts, in accord with their high specific star-formation rates. These results demonstrate the feasibility of using auroral [O II] measurements for accurate metallicity studies at high redshift in a higher metallicity regime previously unexplored with the direct method in distant galaxies. These results also highlight the difficulty in obtaining the measurements required for direct-method metallicities from the ground. We emphasize the advantages that the JWST/NIRSpec instrument will bring to high-redshift metallicity studies, where the combination of increased sensitivity and uninterrupted wavelength coverage will yield more than an order of magnitude increase in efficiency for multiplexed auroral-line surveys relative to current ground-based facilities. Consequently, the advent of JWST promises to be the beginning of a new era of precision chemical abundance studies of the early universe at a level of detail rivaling that of local galaxy studies.