Active Galactic Nuclei with a Low-Metallicity Narrow-Line Region
  Low-metallicity active galactic nuclei (AGNs) are interesting to study the
early phase of the 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]$\lambda$5007/H$\beta$$\lambda4861$ and [N
II]$\lambda6584$/H$\alpha$$\lambda6563$ flux ratios. Specifically,
photoionization models predict that low-metallicity AGNs show a high [O
III]$\lambda$5007/H$\beta$$\lambda$4861 ratio and a relatively low [N
II]$\lambda$6584/H$\alpha$$\lambda$6563 ratio, that corresponds to the location
between the sequence of star-forming galaxies and that of usual AGNs on the BPT
diagram (hereafter "the BPT valley"). However, other populations of galaxies
such as star-forming galaxies and AGNs with a high electron density or a high
ionization parameter could be also located in the BPT valley, not only
low-metallicity AGNs. In this paper, we examine whether most of emission-line
galaxies at the BPT valley are low-metallicity AGNs or not. We select 70
BPT-valley objects from 212,866 emission line galaxies obtained by the Sloan
Digital Sky Survey. Among the 70 BPT-valley objects, 43 objects show firm
evidence of the AGN activity; i.e., the He II$\lambda$4686 emission and/or weak
but significant broad H$\alpha$ 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.
