Galaxy emission line classification using 3D line ratio diagrams

TL;DR

This paper introduces innovative 3D line ratio diagnostic diagrams for galaxy emission classification, enhancing the ability to distinguish between HII regions and AGNs, and providing probabilistic assessments of galaxy types.

Contribution

The authors develop and validate 3D diagnostic diagrams (ZQE) and derived 2D ZE diagnostics, offering improved galaxy classification and insights into excitation mechanisms compared to traditional methods.

Findings

ZE diagnostics effectively separate HII regions from AGNs.

ZE diagnostics are consistent with existing log[NII]/Hα vs. log[OIII]/Hβ diagnostics.

ZQE diagrams enable probabilistic classification of galaxy excitation mechanisms.

Abstract

Two-dimensional (2D) line ratio diagnostic diagrams have become a key tool in understanding the excitation mechanisms of galaxies. The curves used to separate the different regions - HII-like or else excited by an active galactic nucleus (AGN) - have been refined over time but the core technique has not evolved significantly. However, the classification of galaxies based on their emission line ratios really is a multi-dimensional problem. Here we exploit recent software developments to explore the potential of three-dimensional (3D) line ratio diagnostic diagrams. We introduce a specific set of 3D diagrams, the ZQE diagrams, which separate the oxygen abundance and the ionisation parameter of HII region-like spectra, and which also enable us to probe the excitation mechanism of the gas. By examining these new 3D spaces interactively, we define a new set of 2D diagnostics, the ZE diagnostics, which can provide the metallicity of objects excited by hot young stars, and which cleanly separate HII region-like objects from the different classes of AGNs. We show that these ZE diagnostics are consistent with the key log[NII]/H$\alpha$ vs. log[OIII]/H$\beta$ diagnostic currently used by the community. They also have the advantage of attaching a probability that a given object belongs to one class or to the other. Finally, we discuss briefly why ZQE diagrams can provide a new way to differentiate and study the different classes of AGNs in anticipation of a dedicated follow-up study.