Constraining Photoionization Models With a Reprojected Optical Diagnostic Diagram

TL;DR

This paper introduces a reprojected 3D optical diagnostic diagram that enhances the ability to constrain photoionization models, distinguish ionization sources, and address classification ambiguities in galaxy emission line analysis.

Contribution

The authors present a novel reprojection of the 3D line ratio space that improves model constraints and clarifies the classification of ionized regions in galaxies.

Findings

Reprojection reveals model-data discrepancies in line ratios.

A single nitrogen prescription cannot fit both star-forming and AGN loci.

The method allows estimation of AGN contributions in composite regions.

Abstract

Optical diagnostic diagrams are powerful tools to separate different ionizing sources in galaxies. However, the model-constraining power of the most widely-used diagrams is very limited and challenging to visualize. In addition, there have always been classification inconsistencies between diagrams based on different line ratios, and ambiguities between regions purely ionized by active galactic nuclei (AGNs) and composite regions. We present a simple reprojection of the 3D line ratio space composed of [N II]$\lambda 6583$/H$\alpha$, [S II]$\lambda \lambda$6716, 6731/H$\alpha$, and [O III]$\lambda$ 5007/H$\beta$, which reveals its model-constraining power and removes the ambiguity for the true composite objects. It highlights the discrepancy between many theoretical models and the data loci. With this reprojection, we can put strong constraints on the photoionization models and the secondary nitrogen abundance prescription. We find that a single nitrogen prescription cannot fit both the star-forming locus and AGN locus simultaneously, with the latter requiring higher N/O ratios. The true composite regions stand separately from both models. We can compute the fractional AGN contributions for the composite regions, and define demarcations with specific upper limits on contamination from AGN or star formation. When the discrepancy about nitrogen prescriptions gets resolved in the future, it would also be possible to make robust metallicity measurements for composite regions and AGNs.