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

TL;DR

This paper demonstrates that applying the t-SNE technique to SDSS galaxy data enables clear separation of type-2 AGN and HII galaxies in BPT diagrams, improving classification accuracy and aligning observations with theoretical models.

Contribution

The study introduces a novel application of t-SNE to distinguish galaxy types in BPT diagrams, providing a robust method for defining dividing lines based on real data.

Findings

t-SNE effectively separates galaxy types in BPT diagrams

Mathematically determined dividing lines match theoretical expectations

Enhanced classification of composite galaxies in BPT diagrams

Abstract

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$\alpha$.