Powerful t-SNE technique leading to clear separation of type-2 AGN and
  HII galaxies in BPT diagrams
  Narrow emission-line galaxies can be distinguished in the well-known BPT
diagrams through narrow emission line properties. However, there are no
boundaries visible to the naked eye between type-2 AGN and HII galaxies in BPT
diagrams, besides the extreme dividing lines expected by theoretical
photoionization models. Here, based on powerful t-SNE technique applied to the
local narrow emission-line galaxies in SDSS DR15, type-2 AGN and HII galaxies
can 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 the
actual results from real observed properties. The results not only provide an
interesting and robust method to determine the dividing lines in BPT diagrams
through the powerful t-SNE technique, but also lead to further confirmation on
previously defined composite galaxies more efficiently classified in the BPT
diagram of [OIII]/H$\beta$ versus [NII]/H$\alpha$.
