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

Christopher J. Agostino, Samir Salim+7 more|arXiv (Cornell University)|Aug 17, 2021

Astrophysics and Star Formation StudiesPhysics and Astronomy164 references32 citations

TL;DR

This study removes star formation contamination from emission lines in ~100,000 SDSS Seyfert 2s and LINERs using the doppelganger method, revealing that intrinsic physical diversity—particularly in ionization parameter, metallicity, and radiation field hardness—drives the observed line ratio spread on the [NII] BPT diagram, not mixing with star-forming regions. It identifies two distinct LINER populations: soft and hard, with Seyfert 2s differing primarily in higher ionization parameter.

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 star-forming 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.

Motivation & Objective

• To isolate intrinsic emission line ratios in Seyfert 2s and LINERs by removing contamination from star formation.
• To determine whether the extended morphology of the [NII] BPT branch is due to mixing with star-forming regions or intrinsic physical diversity.
• To classify LINERs into distinct populations based on emission line properties after decontamination.
• To assess the role of ionization field hardness and host galaxy properties in shaping observed line ratios.

Proposed method

• Empirically removes star formation contributions using the doppelganger method, matching galaxies by stellar population and dust content.
• Applies k-means clustering to seven decontaminated emission line ratios to identify intrinsic subtypes.
• Uses model grids to interpret the dependence of line ratios on ionization parameter (U) and metallicity (Z).
• Analyzes the relationship between ionization field hardness and host galaxy properties (sSFR, M*) using color-coded diagrams.
• Compares [NII], [SII], and [OI] diagnostics to evaluate their effectiveness in identifying LINERs after SF removal.
• Validates results by comparing intrinsic line ratios to observed values and assessing shifts due to contamination.

Experimental results

Research questions

• RQ1 Is the extended morphology of the [NII] BPT branch in Seyfert 2s and LINERs primarily due to mixing with star-forming regions or intrinsic physical diversity?
• RQ2 What physical parameters—ionization parameter, metallicity, radiation field hardness—best explain the intrinsic line ratio diversity in Seyfert 2s and LINERs?
• RQ3 Do LINERs form a single population or multiple subtypes with distinct ionizing sources?
• RQ4 How do [NII], [SII], and [OI] diagnostics compare in identifying LINERs after removing star formation contamination?
• RQ5 What is the relationship between ionization field hardness and host galaxy properties such as sSFR and stellar mass?

Key findings

• The observed spread along the [NII] BPT branch is primarily due to intrinsic physical diversity in ionization parameter and metallicity, not contamination from star formation.
• After removing star formation, the morphology of the Seyfert/LINER branch remains largely unchanged, indicating intrinsic spread is dominant.
• Seyfert 2s and LINERs exhibit a wide range of radiation field hardness, which cannot be captured by 2-parameter models (U and Z) alone.
• LINERs consist of two distinct populations: soft LINERs (lower hardness) and hard LINERs (higher hardness), with the latter resembling Seyfert 2s more closely in ionization properties.
• The [NII] BPT diagram is more effective than [SII] or [OI] diagnostics for identifying LINERs, especially soft LINERs, even after SF removal.
• Hosts of soft and hard LINERs span a wide range of sSFRs, while Seyfert 2s and soft LINERs are predominantly star-forming, suggesting transient or evolving ionizing mechanisms.