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

Abstract

In the manuscript, we discuss properties of the SDSS J1042-0018 which is a broad line AGN but mis-classified as a HII galaxy in the BPT diagram (SDSS J1042-0018 called as a mis-classified broad line AGN). The emission lines around H$\alpha$ and around H$\beta$ 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 the SDSS J1042-0018 as a HII galaxy in the BPT diagram. In order to explain the unique properties of the mis-classified broad line AGN SDSS J1042-0018, two methods are proposed, the starforming contributions and the compressed NLRs with high electron densities near to critical densities of forbidden emission lines. Fortunately, the strong starforming contributions can be preferred in the SDSS J1042-0018. The mis-classified broad line AGN SDSS J1042-0018, well explained by starforming contributions, could provide further clues on the applications of BPT diagrams to the normal broad line AGN.