Crossing the Line: Active Galactic Nuclei in the Star-forming Region of the BPT Diagram

Abstract

Abstract In this work, we investigate the reliability of the BPT diagram for excluding galaxies that host an active galactic nucleus (AGN). We determine the prevalence of X-ray AGNs in the star-forming region of the BPT diagram and discuss the reasons behind this apparent misclassification, focusing primarily on relatively massive ( ) galaxies. X-ray AGNs are selected from deep XMM observations using a new method that results in greater samples with a wider range of X-ray luminosities, complete to for z < 0.3. Taking X-ray detectability into account, we find that the average fraction of X-ray AGNs in the BPT star-forming branch is 2%, suggesting the BPT diagram can provide a reasonably clean sample of star-forming galaxies. However, the X-ray selection is itself rather incomplete. At the tip of the AGN branch of the BPT diagram, the X-ray AGN fraction is only 14%, which may have implications for studies that exclude AGNs based only on X-ray observations. Interestingly, the X-ray AGN fractions are similar for Seyfert and LINER populations, consistent with LINERs being true AGNs. We find that neither the star formation dilution nor the hidden broad-line components can satisfactorily explain the apparent misclassification of X-ray AGNs. On the other hand, ∼40% of all X-ray AGNs have weak emission lines such that they cannot be placed on the BPT diagram at all and often have low specific SFRs. Therefore, the most likely explanation for “misclassified” X-ray AGNs is that they have intrinsically weak AGN lines, and are only placeable on the BPT diagram when they tend to have high specific SFRs.