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

Abstract

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.

A New Physical Picture for Active Galactic Nuclei Lacking Optical Emission Lines

“…When the S/N in Hβ is relatively high (>10), we use the Balmer decrement (assuming Hα/Hβ = 3.1 and the Cardelli et al 1989 extinction curve) method to correct for the dust extinction. Otherwise, when Hβ S/N is lower, resulting in a rather uncertain Balmer decrement, we follow Agostino et al (2021), where the Balmer decrement is estimated from A V (stellar continuum dust attenuation). The estimation is based on the relationship between the Balmer decrement and A V for objects with well determined Balmer decrement (S/N in Hβ > 10).…”

A New Infrared Criterion for Selecting Active Galactic Nuclei to Lower Luminosities

“…It is of interest to consider galaxies that lie within the composite region of the BPT diagram, as these objects may be AGNs. This is especially relevant as BPT composite galaxies may contain lowerluminosity AGNs where the emission signature is contaminated more strongly by star formation, or comprises interesting subclasses of low ionization nuclear emission region galaxies (Agostino et al 2021). In this section, we investigate how drastically our selection criterion would change with the inclusion of BPT composite galaxies as a target class.…”

A New Infrared Criterion for Selecting Active Galactic Nuclei to Lower Luminosities

“…The Software: Astropy (Collaboration et al 2013), GELATO (Hviding 2022), L A T E X (Lamport 1994), Matplotlib (Hunter 2007), NumPy (Oliphant 2006;van der Walt et al 2011;Harris et al 2020), SciPy (Virtanen et al 2020), SpectRes (Carnall 2017) APPENDIX A. BPT COMPOSITE GALAXIES It is of interest to consider galaxies which lie within the composite region of the BPT diagram as these objects may be AGNs. This is especially relevant as BPT composite galaxies may contain lower luminosity AGN where the emission signature contaminated more strongly by star formation, or be comprised of interesting subclasses of Low Ionization Nuclear Emission Regions galaxies (Agostino et al 2021). In this section we investigate how drastically our selection criterion would change with the inclusion of BPT composite galaxies as a target class.…”