We use JWST Near-Infrared Spectrograph (NIRSpec) observations from the the Cosmic Evolution Early Release survey (CEERS), GLASS-JWST ERS (GLASS), and JWST Advanced Deep Extragalactic Survey (JADES) to measure rest-frame optical emission-line ratios of 90 galaxies at z>4. The stacked spectra of galaxies with and without a broad-line feature reveal a difference in the [OIII]λ 4363 and Hγ ratios. This motivated our investigation of the [OIII]/Hγ vs [NeIII]/[OII] diagram. We define two AGN/SF classification lines based on 1869 SDSS galaxies at z∼0. After applying a redshift correction to the AGN/SF lines we find 76.8% of BLAGN continue to land in the AGN region of the diagnostic largely due to the [NeIII]/[OII] ratio. However, 40.2% of non-BLAGN land in the AGN region as well, this could be due to star forming galaxies having harder ionization of there are narrow line AGN which are not accounted for. This indicates the potential of the [NeIII]/[OII] ratio to continue classifying galaxies to z∼6. We further inspect galaxies without broad emission lines in each region of [OIII]/H\gamma vs [NeIII]/[OII] diagram and found that they have slightly stronger CIII]λ1908 fluxes and equivalent width when landing in the BLAGN region. However, the cause of this higher ionization is unclear. Additionally, we find that BLAGN are characterized by a higher ionization (at constant electron temperature) compared to non-broad line galaxies.
@article{arxiv.2502.03519,
title = {Emission-Line Diagnostics at z>4: [OIII]{\lambda}4363/H\gamma},
author = {Bren E. Backhaus and Nikko J. Cleri and Jonathan R. Trump and Allison Kirkpatrick and Raymond C. Simons and Pablo Arrabal Haro and Micaela B. Bagley and Madisyn Brooks and Antonello Calabrò and Kelcey Davis and Mark Dickinson and Steven L. Finkelstein and Michaela Hirschmann and Jeyhan S. Kartaltepe and Anton M. Koekemoer and Mario Llerena and Fabio Pacucci and Nor Pirzkal and Casey Papovich and Stephen M. Wilkins},
journal= {arXiv preprint arXiv:2502.03519},
year = {2025}
}