Beyond diagnostic-diagrams: A critical exploration on the classification of ionization processes

Abstract: Optical emission-line diagnostic diagrams, such as the classical BPT, are widely used to identify ionisation mechanisms in galaxies but often suffer from degeneracies, especially when multiple sources coexist. We critically evaluate the effectiveness of these diagnostics in distinguishing star-forming galaxies, retired galaxies (RGs), and active galactic nuclei (AGNs), and propose refined methods to reduce misclassifications. Using a large sample of nearby galaxies from the NASA-Sloan Atlas cross-matched with SDSS spectroscopy, we define representative subsamples of late-type/star-forming galaxies, early-type/RGs, and multiwavelength-selected AGNs. Their distributions are analysed across traditional and modern diagnostics, including WHaN, WHaD, and the newly introduced WHaO diagram, which combine H{\alpha} equivalent width with [N II]/H{\alpha}, {\sigma}(H{\alpha}), and [O III]/[O II], respectively. Quantitative comparisons reveal that classical BPT diagrams overestimate star-forming galaxies by ~10% and misclassify up to 45% of AGNs and nearly all RGs. Diagnostics incorporating EW(H{\alpha}) achieve improved accuracy, reducing misclassifications to {\sim} 20\ % for AGNs and {\sim} 15\% for RGs. A revised classification scheme based on EW(H{\alpha}) thresholds and consistent WHaD/WHaO results yields the highest purity (8-25% misclassifications) and better reflects underlying physical conditions. Our analysis demonstrates that BPT-based methods fail to reliably separate ionisation mechanisms, particularly in galaxies hosting weak AGNs or evolved stellar populations. Updated EW(H{\alpha})-based diagnostics offer a more accurate framework for studying galaxy ionisation and should replace traditional schemes in future population studies.