Physical Drivers of Emission-line Diversity of SDSS Seyfert 2s and LINERs after Removal of Contributions from Star Formation

Ionization sources other than HII regions give rise to the right-hand branch in standard ([NII]) BPT diagram, populated by Seyfert 2s and LINERs. However, because majority of Seyfert/LINER hosts are star forming (SF), contaminate observed lines some extent, making it unclear if position along is merely due various degrees mixing between pure SF, or whether reflects intrinsic diversity ionizing sources. In this study, we empirically remove SF contributions ~100,000 Seyfert/LINERs from SDSS using doppelganger method. We find that not principal cause extended morphology branch. Rather, Seyferts/LINERs intrinsically have a wide range line ratios. Variations ionization parameter metallicity can account for much ratios, but hardness field also varies significantly. Furthermore, our k-means classification on seven decontaminated emission reveals LINERs made up two populations, which call soft hard The differ both types primarily higher parameter, whereas LINER mainly each (and star-forming regions) radiation field. confirm [NII] diagram more efficiently identifies [SII] [OI] diagnostics, latter many LINERs, especially ones, occupy same location as star-formers, even after has been removed emission.