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In this paper, we discuss properties of SDSS J1042-0018 which is a broad line active galactic nucleus (AGN) but misclassified as an H  ii  galaxy in the BPT diagram (SDSS J1042-0018 is called a misclassified broad line AGN). The emission lines around H α  and around H β  are well described by different model functions, considering broad Balmer lines to be described by Gaussian or Lorentz functions. Different model functions lead to different determined narrow emission line fluxes, but the different narrow emission line flux ratios lead SDSS J1042-0018 as an H  ii  galaxy in the BPT diagram. In order to explain the unique properties of the misclassified broad line AGN SDSS J1042-0018, two methods are proposed, the star-forming contributions and the compressed narrow emission line regions with high electron densities near to critical densities of forbidden emission lines. Fortunately, the strong star-forming contributions can be preferred in SDSS J1042-0018. The misclassified broad line AGN SDSS J1042-0018, well explained by star-forming contributions, could provide further clues on the applications of BPT diagrams to the normal broad line AGNs.

SDSS J1042-0018: a Broad Line AGN but Misclassified as an H ii Galaxy in the BPT Diagram by Flux Ratios of Narrow Emission Lines