[Paper Review] New AGN diagnostic diagrams based on the [OIII]$λ4363$ auroral line

Giovanni Mazzolari, Hannah Übler+9 more|arXiv (Cornell University)|Apr 16, 2024

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TL;DR

The paper introduces three rest-frame optical diagnostic diagrams that use the [OIII]λ4363 auroral line to identify AGN, leveraging JWST-accessible lines and tested with extensive local and high-redshift samples plus photoionization models.

ABSTRACT

The James Webb Space Telescope (JWST) is revolutionizing our understanding of black holes formation and growth in the early Universe. However, JWST has also revealed that some of the classical diagnostics, such as the BPT diagrams and X-ray emission, often fail to identify narrow line TypeII active galactic nuclei (AGN) at high redshift. Here we present three new rest-frame optical diagnostic diagrams leveraging the [OIII]$\lambda4363$ auroral line, which has been detected in several JWST spectra. Specifically, we show that high values of the [OIII]$\lambda4363/$H$γ$ ratio provide a sufficient (but not necessary) condition to identify the presence of an AGN, both based on empirical calibrations (using local and high-redshift sources) and a broad range of photoionization models. These diagnostics are able to separate much of the AGN population from Star Forming Galaxies (SFGs). This is because the average energy of AGN's ionizing photons is higher than that of young stars in SFGs, hence AGN can more efficiently heat the gas, therefore boosting the [OIII]$\lambda4363$ line. We also found independent indications of AGN activity in some high-redshift sources that were not previously identified as AGN with the traditional diagnostics diagrams, but that are placed in the AGN region of the diagnostics presented in this work. We note that, conversely, low values of [OIII]$\lambda4363/$H$γ$ can be associated either with SFGs or AGN excitation. We note that the fact that strong auroral lines are often associated with AGN does not imply that they cannot be used for direct metallicity measurements (provided that proper ionization corrections are applied), but it does affect the calibration of strong line metallicity diagnostics.

Motivation & Objective

• Motivate the need for new AGN diagnostics at high redshift where traditional diagrams fail with JWST data.
• Propose three rest-frame optical diagnostics based on [OIII]4363 and complementary lines to separate AGN from SFGs.
• Calibrate and validate diagnostics against local and high-z samples and against photoionization models.
• Discuss implications for metallicity estimates and ionization conditions in AGN versus star-forming galaxies.

Proposed method

• Compile extensive local and high-z observational samples with detected [OIII]λ4363 fluxes (SFGs and AGN).
• Construct three new diagnostic diagrams combining [OIII]λ4363/Hγ with (a) [OIII]5007/[OII]3727, (b) [NeIII]3869/[OII]3727, and (c) [OIII]5007/[OIII]4363 ratios.
• Compare observational distributions with photoionization model grids (Gutkin+2016, Feltre+2016) and Nakajima & Maiolino (2022) variants.
• Apply likelihood-based cuts in metallicity–ionization parameter space to prune unphysical SFG models.
• Assess reddening effects and the robustness of AGN separation across redshift.

Figure 1 : Diagnostic diagram showing the [OIII] $\lambda 4363\,$ /H $\gamma\ $ vs [OIII] $\lambda 5007\,$ /[OII] $\lambda 3727\,$ lines ratios. The [OII] $\lambda 3727\,$ is the sum of the doublet [OII] $\lambda\lambda 3726,3728$ . Left panel: plot of all the observational samples described in Sec.

Experimental results

Research questions

• RQ1 Can [OIII]λ4363/Hγ, when combined with other near-IR line ratios, reliably separate AGN from star-forming galaxies at low and high redshift?
• RQ2 Do photoionization models reproduce the AGN-laden regions of the new diagrams and distinguish them from SFGs across parameter space?
• RQ3 What is the impact of dust reddening on the placement of galaxies in these diagrams, and can the diagnostics remain robust at high z?
• RQ4 What are the implications of these diagnostics for metallicity measurements using [OIII]4363 and related strong-line calibrations?
• RQ5 Do high-z JWST-detected galaxies not classified as AGN by traditional diagrams occupy the AGN regions in these new diagrams?

Key findings

• Three new diagnostics based on [OIII]λ4363/Hγ with either [OIII]5007/[OII]3727, [NeIII]3869/[OII]3727, or [OIII]5007/[OIII]4363 effectively separate AGN from SFGs in local and high-z samples.
• AGN populations occupy regions inaccessible to pure SFG models, while many SFGs remain excluded from AGN regions across all diagrams.
• Some high-z sources not previously identified as AGN fall into the AGN regions of these diagrams, indicating enhanced AGN indicators in JWST data.
• Auroral [OIII]4363 emission is enhanced in AGN due to harder ionizing spectra, enabling temperature-based metallicity considerations with appropriate ionization corrections.
• Dust reddening can move objects along the diagrams, potentially shifting AGN-dominated sources, but SFGs are less likely to contaminate AGN regions under typical reddening.
• Model comparisons show SFG grids rarely reach AGN regions, while AGN grids cover both regions including those populated by observed AGN.
• The work highlights implications for re-evaluating metallicity calibrations that rely on [OIII]4363, particularly under AGN influence.

Figure 2 : Same as in Fig. 1 , but for the lines ratios [OIII] $\lambda 4363\,$ /H $\gamma\ $ vs [Ne III] $\lambda 3869\ $ /[OII] $\lambda 3727\,$ . Based on the distribution of observational samples and models, also this diagnostic diagram identifies a region that can be populated only by AGN.