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Two-dimensional (2D) line ratio diagnostic diagrams have become a key tool inunderstanding the excitation mechanisms of galaxies. The curves used toseparate the different regions - HII-like or else excited by an active galacticnucleus (AGN) - have been refined over time but the core technique has notevolved significantly. However, the classification of galaxies based on theiremission line ratios really is a multi-dimensional problem. Here we exploitrecent software developments to explore the potential of three-dimensional (3D)line ratio diagnostic diagrams. We introduce a specific set of 3D diagrams, theZQE diagrams, which separate the oxygen abundance and the ionisation parameterof HII region-like spectra, and which also enable us to probe the excitationmechanism of the gas. By examining these new 3D spaces interactively, we definea new set of 2D diagnostics, the ZE diagnostics, which can provide themetallicity of objects excited by hot young stars, and which cleanly separateHII region-like objects from the different classes of AGNs. We show that theseZE diagnostics are consistent with the key log[NII]/H$\alpha$ vs.log[OIII]/H$\beta$ diagnostic currently used by the community. They also havethe advantage of attaching a probability that a given object belongs to oneclass or to the other. Finally, we discuss briefly why ZQE diagrams can providea new way to differentiate and study the different classes of AGNs inanticipation of a dedicated follow-up study.

Galaxy emission line classification using 3D line ratio diagrams