Physical Drivers of Emission Line Diversity of SDSS Seyfert 2s and LINERs After Removal of Contributions by Star Formation

Abstract

Ionization sources other than HII regions give rise to the right-hand branch in the standard ([NII]) BPT diagram, populated by Seyfert 2s and LINERs. However, because the majority of Seyfert/LINER hosts are star forming (SF), HII 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 Seyfert/LINER 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 Seyfert/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 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 starforming regions) in the hardness of the radiation field. We confirm that the [NII] BPT diagram more efficiently identifies LINERs than [SII] and [OI] diagnostics, because in the latter many LINERs, especially soft ones, occupy the same location as pure star-formers, even after the SF has been removed from LINER emission.

Signs of environmental effects on star-forming galaxies in the Spiderweb protocluster at z = 2.16

“…(iv) We extract [N ]-based metallicities for a subsample of 22 HAEs. We find that 5 of them have [N ]/H𝛼 ratios consistent with those of AGN candidates according to Agostino et al (2021). Most of the remaining protocluster members still show individual enhanced metallicities by up to ∼ 0.25 dex with respect to the field massmetallicity relation at similar redshift (Erb et al 2006;Wisnioski et al 2019;Sanders et al 2021).…”

“…Furthermore, we consider the effect of AGN contamination by splitting our sample in objects below and above log([N ]/H𝛼)≥ −0.35 in the same figure (filled and empty circles respectively). This threshold has been recently proposed by Agostino et al (2021) as a modification of the Kauffmann et al (2003) line to separate the purely star-forming from the composite and AGN regions in the BPT diagram (Baldwin et al 1981) at relatively low [O ]/H𝛽 ratios (e.g. log([O ]/H𝛽) ≤ −0.2).…”

Chemical abundances in Seyfert galaxies -IX. Helium abundance estimates

“…In fact, most of the AGN studies have relied on large surveys just to address the oxygen abundance and/or physical properties (e.g., ionization degree, electron temperature, electron density) of these objects (e.g. Groves et al 2006;Zhang et al 2008;Vaona et al 2012;Ludwig et al 2012;Zhang et al 2013;Richardson et al 2014;Coil et al 2015;Feltre et al 2016;Kawasaki et al 2017;Carvalho et al 2020;Dors et al 2020a;Dors 2021;Agostino et al 2021). The abundances of other elements beyond the oxygen are poorly known in AGNs, in particular, the local Seyfert 2, for which large amount of spectroscopic data are available in the literature.…”

Photometric variability in star-forming galaxies as evidence for low-mass AGN and a precursor to quenching

“…Generally, commonly used indicators cannot be uniformly applied to all galaxies with equal effectiveness. For instance, the oftenused practise of measuring emission line ratios to distinguish AGN emission from star-formation is often not effective in strongly starforming galaxies because the star-formation swamps the AGN emission (Baldwin et al 1981;Cann et al 2019;Agostino et al 2021). One promising route to a uniform indicator is through examining the photometric variability.…”