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Low-metallicity active galactic nuclei (AGNs) are interesting to study theearly phase of the AGN evolution. However most AGNs are chemically matured andaccordingly low-metallicity AGNs are extremely rare. One approach to search forlow-metallicity AGNs systematically is utilizing the so-called BPT diagram thatconsists of the [O III]$\lambda$5007/H$\beta$$\lambda4861$ and [NII]$\lambda6584$/H$\alpha$$\lambda6563$ flux ratios. Specifically,photoionization models predict that low-metallicity AGNs show a high [OIII]$\lambda$5007/H$\beta$$\lambda$4861 ratio and a relatively low [NII]$\lambda$6584/H$\alpha$$\lambda$6563 ratio, that corresponds to the locationbetween the sequence of star-forming galaxies and that of usual AGNs on the BPTdiagram (hereafter "the BPT valley"). However, other populations of galaxiessuch as star-forming galaxies and AGNs with a high electron density or a highionization parameter could be also located in the BPT valley, not onlylow-metallicity AGNs. In this paper, we examine whether most of emission-linegalaxies at the BPT valley are low-metallicity AGNs or not. We select 70BPT-valley objects from 212,866 emission line galaxies obtained by the SloanDigital Sky Survey. Among the 70 BPT-valley objects, 43 objects show firmevidence of the AGN activity; i.e., the He II$\lambda$4686 emission and/or weakbut significant broad H$\alpha$ emission. Our analysis shows that those 43BPT-valley AGNs are not characterized by a very high gas density nor ionizationparameter, 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 efficienttool to search for low-metallicity AGNs.

Active Galactic Nuclei with a Low-Metallicity Narrow-Line Region