2017ApJ...842...44K -
Astrophys. J., 842, 44-44 (2017/June-2)
Active galactic nuclei with a low-metallicity narrow-line region.
KAWASAKI K., NAGAO T., TOBA Y., TERAO K. and MATSUOKA K.
Abstract (from CDS):
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/Hβλ4861 and [N II]λ6584/Hαλ6563 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 IIλ4686 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.
Abstract Copyright:
© 2017. The American Astronomical Society. All rights reserved.
Journal keyword(s):
galaxies: abundances - galaxies: active - galaxies: ISM - galaxies: nuclei - galaxies: Seyfert - galaxies: Seyfert
Errata:
erratum vol. 854, art. 76 (2018)
Simbad objects:
72
Full paper
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