Crossing the Line: Active Galactic Nuclei in the Star-forming region of the BPT Diagram. (arXiv:1811.11755v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Agostino_C/0/1/0/all/0/1">Christopher J. Agostino</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Salim_S/0/1/0/all/0/1">Samir Salim</a>

In this work, we investigate the reliability of the BPT diagram for excluding
the galaxies that host an AGN. Specifically, we determine the prevalence of
X-ray AGN in the star-forming region of the BPT diagram and discuss the reasons
behind this apparent misclassification. X-ray AGN are selected from very deep
XMM observations using a new method that results in greater samples with a
wider range of X-ray luminosities, complete to $log(L_{X})>41$ for $z<0.3$. Taking X-ray detectability into account, we find that the fraction of X-ray AGN in the star-forming branch of the BPT diagram is 2% on average, suggesting that optical emission-line selection via 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 ~15%, likely due to the obscurationx of X-rays. This has implications for high-redshift studies that exclude AGN based only on X-ray observations. Interestingly, the X-ray AGN fractions are similar for Seyfert and LINER populations, consistent with LINERs being true AGN. We find that neither the star-formation dilution nor hidden broad-line components can satisfactorily explain the apparent misclassification of X-ray AGN in the star-forming region of the BPT diagram. On the other hand, ~40% of all X-ray AGN have very weak emission lines such that they cannot be placed on the BPT diagram at all. They also tend to have very low specific SFRs. Therefore, the most likely explanation for the X-ray AGN found in the star-forming region of the BPT diagram is that they have intrinsically weak AGN lines, and are only classifiable by the BPT diagram when they tend to have high specific SFRs.

In this work, we investigate the reliability of the BPT diagram for excluding
the galaxies that host an AGN. Specifically, we determine the prevalence of
X-ray AGN in the star-forming region of the BPT diagram and discuss the reasons
behind this apparent misclassification. X-ray AGN are selected from very deep
XMM observations using a new method that results in greater samples with a
wider range of X-ray luminosities, complete to $log(L_{X})>41$ for $z<0.3$.
Taking X-ray detectability into account, we find that the fraction of X-ray AGN
in the star-forming branch of the BPT diagram is 2% on average, suggesting that
optical emission-line selection via 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 ~15%, likely due to the obscurationx of X-rays. This has
implications for high-redshift studies that exclude AGN based only on X-ray
observations. Interestingly, the X-ray AGN fractions are similar for Seyfert
and LINER populations, consistent with LINERs being true AGN. We find that
neither the star-formation dilution nor hidden broad-line components can
satisfactorily explain the apparent misclassification of X-ray AGN in the
star-forming region of the BPT diagram. On the other hand, ~40% of all X-ray
AGN have very weak emission lines such that they cannot be placed on the BPT
diagram at all. They also tend to have very low specific SFRs. Therefore, the
most likely explanation for the X-ray AGN found in the star-forming region of
the BPT diagram is that they have intrinsically weak AGN lines, and are only
classifiable by the BPT diagram when they tend to have high specific SFRs.

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