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Emission-line Diagnostics at z > 4: O iiiλ4364/Hγ versus Ne iiiλ3870/O iiλ3730
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Emission-line Diagnostics at z > 4: O iiiλ4364/Hγ versus Ne iiiλ3870/O iiλ3730
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Journal Article
Emission-line Diagnostics at z > 4: O iiiλ4364/Hγ versus Ne iiiλ3870/O iiλ3730
2025
Overview
We use JWST Near-Infrared Spectrograph observations from the Cosmic Evolution Early Release survey, GLASS-JWST ERS (GLASS), and JWST Advanced Deep Extragalactic Survey to measure rest-frame optical emission-line ratios of 89 galaxies at z > 4. The stacked spectra of galaxies with and without a broad-line feature reveal a difference in the [O iii]λ 4364 and Hγ ratios. This motivated our investigation of the [O iii]λ4364/Hγ versus [Ne iii]/[O ii] diagram. We define two active galactic nucleus (AGN)/star formation (SF) classification lines based on 21,048 Sloan Digital Sky Survey galaxies at z ∼ 0. After applying a redshift correction to the AGN/SF lines, we find 69.2% of broad-line active galactic nuclei (BLAGN) continue to land in the AGN region of the diagnostic, largely due to the [Ne iii]/[O ii] ratio. However, 33.0% of non-BLAGN land is in the AGN region as well. The [O iii]λ4364/Hγ versus [Ne iii]/[O ii] diagram does not robustly separate BLAGN from non-broad-line galaxies at z > 4. This could be due to star-forming galaxies having harder ionization, or these galaxies contain a narrow line AGN, which are not accounted for. We further inspected galaxies without broad emission lines in each region of [O iii]λ4364/Hγ versus [Ne iii]/[O ii] diagram and found that they have slightly stronger C iii]λ1908 fluxes and equivalent width when landing in the BLAGN region. However, the cause of this higher ionization is unclear and may be revealed by observing UV lines.
Publisher
IOP Publishing
