Active Galactic Nuclei with a Low-metallicity Narrow-line Region

Kawasaki, Kota; Nagao, Tohru; Toba, Yoshiki; Terao, Koki; Matsuoka, Kenta

doi:10.3847/1538-4357/aa70e1

Cited by 20 publications

(21 citation statements)

References 53 publications

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“…Low-metallicity AGN should lie between the 'wings' of the BPT 'butterfly diagram' (e.g. Kawasaki et al 2017); in our sample objects in this region appear to have a combination of high ionization parameter and relatively low metallicity (Figure 1).…”

Section: Further Analysis and Discussionmentioning

confidence: 78%

The Mass–Metallicity Relation of Local Active Galaxies

Childress

Kewley

Dopita

et al. 2019

ApJ

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We systematically measure the gas-phase metallicities and the mass-metallicity relation of a large sample of local active galaxies for the first time. Observed emission-line fluxes from the Sloan Digital Sky Survey (SDSS) are compared to a four-dimensional grid of photoionization models using the Bayesian parameter estimation code NebulaBayes. For the first time we take into account arbitrary mixing between H II region and narrow-line region (NLR) emission, and the models are also varied with metallicity, ionization parameter in the NLR, and the gas pressure. The active galactic nucleus (AGN) oxygen abundance is found to increase by ∆O/H ∼ 0.1 dex as a function of host galaxy stellar mass over the range 10.1 < log M * /M < 11.3. We also measure the metallicity and ionization parameter of 231000 star-forming galaxies for comparison with the sample of 7670 Seyfert 2 galaxies. A systematic offset in oxygen abundance of 0.09 dex is observed between the mass-metallicity relations of the star-forming and active galaxies. We investigate potential causes of the offset, including sample selection and the treatment in the models of diffuse ionized gas, pressure, and ionization parameter. We cannot identify the major cause(s), but suspect contributions due to deficiencies in modeling the ionizing spectra and the treatment of dust physics. Optical diagnostic diagrams are presented with the star-forming and Seyfert data colored by the inferred oxygen abundance, ionization parameter and gas pressure, clearly illustrating the trends in these quantities.

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Section: Further Analysis and Discussionmentioning

confidence: 78%

The Mass–Metallicity Relation of Local Active Galaxies

Childress

Kewley

Dopita

et al. 2019

ApJ

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“…We do not see as strong of a dependence for the Sy2s and the log(O/H) values for all three groups are higher than the pure counterparts, likely because of the additional contributions from star-formation to the [NII] and [OII] lines used in the calculation of log(O/H)+12. It is possible that the proposed lack of metal-poor AGNs (e.g., Groves et al (2006), Kawasaki et al (2017)) is in part a result of an inability to remove SF contributions.…”

Section: Appendix a Alternative Matching Without A S/n Requirement On...mentioning

confidence: 99%

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

Agostino,

Salim,

Faber

et al. 2021

Preprint

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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.

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“…Despite the recent progress, measurements of NLR abundances remain less routine than is the case for H IIregion measurements. An example of the ongoing challenges in measuring NLR abundances is the recent work of Kawasaki et al (2017), who investigated using optical diagnostic diagrams to identify low-metallicity AGN, but did not attempt to measure the metallicity of the objects in their sample.…”

Section: Introductionmentioning

confidence: 99%

Interrogating Seyferts with NebulaBayes: Spatially Probing the Narrow-line Region Radiation Fields and Chemical Abundances

Childress

Dopita

Kewley

et al. 2018

ApJ

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NebulaBayes is a new Bayesian code that implements a general method of comparing observed emission-line fluxes to photoionization model grids. The code enables us to extract robust, spatially resolved measurements of abundances in the extended narrow line regions (ENLRs) produced by Active Galactic Nuclei (AGN). We observe near-constant ionization parameters but steeply radially-declining pressures, which together imply that radiation pressure regulates the ENLR density structure on large scales. Our sample includes four 'pure Seyfert' galaxies from the S7 survey that have extensive ENLRs. NGC 2992 shows steep metallicity gradients from the nucleus into the ionization cones. An inverse metallicity gradient is observed in ESO 138-G01, which we attribute to a recent gas inflow or minor merger. A uniformly high metallicity and hard ionizing continuum are inferred across the ENLR of Mrk 573. Our analysis of IC 5063 is likely affected by contamination from shock excitation, which appears to soften the inferred ionizing spectrum. The peak of the ionizing continuum E peak is determined by the nuclear spectrum and the absorbing column between the nucleus and the ionized nebula. We cannot separate variation in this intrinsic E peak from the effects of shock or H II region contamination, but E peak measurements nevertheless give insights into ENLR excitation. We demonstrate the general applicability of NebulaBayes by analyzing a nuclear spectrum from the non-active galaxy NGC 4691 using a H II region grid. The NLR and H II region model grids are provided with NebulaBayes for use by the astronomical community.

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Active Galactic Nuclei with a Low-metallicity Narrow-line Region

Cited by 20 publications

References 53 publications

The Mass–Metallicity Relation of Local Active Galaxies

The Mass–Metallicity Relation of Local Active Galaxies

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

Interrogating Seyferts with NebulaBayes: Spatially Probing the Narrow-line Region Radiation Fields and Chemical Abundances

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