SDSS J1451+2709 a normal blue quasar but mis-classified as a HII galaxy in the BPT diagram by flux ratios of narrow emission lines

TL;DR

This study examines SDSS J1451+2709, a blue quasar misclassified as an HII galaxy in the BPT diagram, exploring its emission line properties and the reasons behind its unique classification.

Contribution

It proposes two methods to explain the misclassification but finds they are insufficient, highlighting the need for further investigation into the physical origins.

Findings

Different emission line models affect classification

No significant long-term variability observed

Potential for narrow-line HII galaxies to host AGN activity

Abstract

In the manuscript, we discuss properties of SDSS J1451+2709, a normal blue quasar but mis-classified as a HII galaxy in the BPT diagram (called as a mis-classified quasar). The emission lines around H$\alpha$ and H$\beta$ are well measured by different model functions with broad Balmer lines being 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 fluxes of narrow emission lines, but the different narrow emission line flux ratios lead the SDSS J1451+2709 to be classified as a HII 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 HII galaxies actually harbouring central AGN activities.