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Narrow emission-line galaxies can be distinguished in the well-known BPTdiagrams through narrow emission line properties. However, there are noboundaries visible to the naked eye between type-2 AGN and HII galaxies in BPTdiagrams, besides the extreme dividing lines expected by theoreticalphotoionization models. Here, based on powerful t-SNE technique applied to thelocal narrow emission-line galaxies in SDSS DR15, type-2 AGN and HII galaxiescan be clearly separated in the t-SNE determined two-dimensional projected map,and then the dividing lines can be mathematically determined in BPT diagrams,leading to charming harmonization of the theoretical expectations and theactual results from real observed properties. The results not only provide aninteresting and robust method to determine the dividing lines in BPT diagramsthrough the powerful t-SNE technique, but also lead to further confirmation onpreviously defined composite galaxies more efficiently classified in the BPTdiagram of [OIII]/H$\beta$ versus [NII]/H$\alpha$.

Powerful t-SNE technique leading to clear separation of type-2 AGN and HII galaxies in BPT diagrams