2022MNRAS.509.4626Z

In the manuscript, we discuss properties of SDSS J1451 + 2709, a normal blue quasar but mis-classified as a H II galaxy in the BPT diagram (called as a mis-classified quasar). The emission lines around H α and around H β are well measured by different model functions with broad Balmer lines described by Gaussian or Lorentz functions, in the SDSS spectrum in 2007 and in the KPNO spectrum in 1990. After considering variations of broad emission lines, different model functions lead to different determined fluxes of narrow emission lines, but the different narrow emission line flux ratios lead the SDSS J1451 + 2709 as a H II galaxy in the BPT diagram. In order to explain the unique properties of the mis-classified quasar SDSS J1451+2709 in the BPT diagram, two methods are proposed, the starforming contributions and compressed NLRs with high electron densities near to critical densities of forbidden emission lines. Unfortunately, the two methods cannot be preferred in the SDSS J1451+2709, further efforts are necessary to find the physical origin of the unique properties of the mis-classified quasar SDSS J1451+2709 in the BPT diagram. Meanwhile, there are not quite different long-term variabilities of SDSS J1451+2709 from the normal quasars. The mis-classified quasar SDSS J1451 + 2709, an extremely unique case or a special case among the normal quasars, could provide further clues on the applications of BPT diagrams to the normal broad line AGN and to narrow emission line objects, indicating part of narrow emission line H II galaxies actually harbouring central AGN activities.