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

In an effort to probe the origin of surface brightness profile (SBP) breaks widely observed in nearby disk galaxies, we carry out a comparative study of stellar population profiles of 635 disk galaxies selected from the Mapping Nearby Galaxies at Apache Point Observatory spectroscopic survey. We classify our galaxies into single exponential (Ti), down-bending (Tii), and up-bending (Tiii) SBP types and derive their spin parameters and radial profiles of age/metallicity-sensitive spectral features. Most Tii (Tiii) galaxies have down-bending (up-bending) star formation rate (SFR) radial profiles, implying that abrupt radial changes of SFR intensities contribute to the formation of both Tii and Tiii breaks. Nevertheless, a comparison between our galaxies and simulations suggests that stellar migration plays a significant role in weakening down-bending Σ⋆ profile breaks. While there is a correlation between the break strengths of SBPs and age/metallicity-sensitive spectral features for Tii galaxies, no such correlation is found for Tiii galaxies, indicating that stellar migration may not play a major role in shaping Tiii breaks, as is also evidenced by a good correspondence between the break strengths of Σ⋆ and SBPs of Tiii galaxies. We do not find evidence for galaxy spin being a relevant parameter for forming different SBP types, nor do we find significant differences between the asymmetries of galaxies with different SBP types, suggesting that environmental disturbances or satellite accretion in the recent past do not significantly influence the break formation. By dividing our sample into early and late morphological types, we find that galaxies with different SBP types follow nearly the same tight stellar mass– relation, which makes the hypothesis that stellar migration alone can transform SBP types from Tii to Ti and then to Tiii highly unlikely.

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The scaling relationship is a fundamental probe of the evolution of galaxies. Using the integral field spectroscopic data from the Mapping Nearby Galaxies at Apache Point Observatory survey, we select 1698 spaxels with a significant detection of the auroral emission line [O iii]λ4363 from 52 galaxies to investigate the scaling relationships at the low-metallicity end. We find that our sample’s star formation rate is higher and its metallicity is lower in the scaling relationship than the star-forming sequence after removing the contribution of the Fundamental Metallicity Relation. We also find that the stellar ages of our sample are younger (<1 Gyr) and the stellar metallicities are also lower. Morphological parameters from the Deep Learning catalog indicate that our galaxies are more likely to be mergers. These results suggest that their low-metallicity regions may be related to interaction; the inflow of metal-poor gas may dilute the interstellar medium and form new metal-poor stars in these galaxies during interactions.

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