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

Journal Article · Sat Jun 10 04:00:00 EDT 2017 · Astrophysical Journal

DOI:https://doi.org/10.3847/1538-4357/AA70E1· OSTI ID:22872632

Kawasaki, Kota; Terao, Koki ^[1]; Nagao, Tohru ^[2]; Toba, Yoshiki ^[3]; Matsuoka, Kenta ^[4]

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1. Department of Physics, Graduate School of Science and Engineering, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577 (Japan)
2. Research Center for Space and Cosmic Evolution, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577 (Japan)
3. Academia Sinica Institute of Astronomy and Astrophysics, P.O. Box 23-141, Taipei 10617, Taiwan (China)
4. Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502 (Japan)

Low-metallicity active galactic nuclei (AGNs) are interesting to study for the early phase of AGN evolution. However, most AGNs are chemically matured, and accordingly, low-metallicity AGNs are extremely rare. One approach to search for low-metallicity AGNs systematically is utilizing the so-called BPT diagram that consists of the [O iii]λ5007/Hβ λ4861 and [N ii]λ6584/Hα λ6563 flux ratios. Specifically, photoionization models predict that low-metallicity AGNs show a high [O iii]λ5007/Hβλ4861 ratio and a relatively low [N ii]λ6584/Hαλ6563 ratio that corresponds to the location between the sequence of star-forming galaxies and that of usual AGNs on the BPT diagram (hereafter “the BPT valley”). However, other populations of galaxies such as star-forming galaxies and AGNs with a high electron density or a high-ionization parameter could be also located in the BPT valley, not only low-metallicity AGNs. In this paper, we examine whether most of the emission-line galaxies at the BPT valley are low-metallicity AGNs or not. We select 70 BPT-valley objects from 212,866 emission-line galaxies obtained by the Sloan Digital Sky Survey. Among the 70 BPT-valley objects, 43 objects show firm evidence of the AGN activity, i.e., the He ii λ4686 emission and/or weak but significant broad Hα emission. Our analysis shows that those 43 BPT-valley AGNs are not characterized by a very high gas density nor ionization parameter, inferring that at least 43 among 70 BPT-valley objects (i.e., >60%) are low-metallicity AGNs. This suggests that the BPT diagram is an efficient tool to search for low-metallicity AGNs.

• OSTI ID: 22872632
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 842; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
DENSITY
ELECTRON DENSITY
EMISSION
GALACTIC EVOLUTION
GALAXY NUCLEI
METALLICITY
PHOTOIONIZATION
SEYFERT GALAXIES
STARS