Abstract
In order to accurately study topics in galaxy evolution, it is necessary to be able to reliably determine which galaxies do or do not host an AGN. Through optical emission line selection with the BPT diagnostic diagram, AGNs and non-AGNs can be studied statistically with large samples from SDSS, but the completeness of the AGN selection and the extent to which the non-AGN selection may be contaminated are not well-parameterized. We place X-ray selected AGNs on the BPT diagram and determine the X-ray AGN fraction as a function of position on the BPT diagram. We explore the possible reasons for the misclassification of some X-ray AGNs, finding that dilution by star formation cannot provide a satisfactory explanation. Motivated by this finding, we then explore what physical factors drive the observed emission line diversity of SDSS AGNs. It has been suggested that the AGN branch of the BPT diagram is a mixing sequence between star formation and AGN emissions, and we test the merits of this scenario by matching AGNs to non-AGN doppelgangers and empirically remove the host contributions to the AGN emission lines. We find that mixing cannot explain the extent of the pure AGN branch. Instead, variations in the ionization and parameter and metallicity are sufficient to explain the apparent diversity of AGN emission lines.
