Emission line properties of Pure AGNs: Investigating the range of positions occupied by AGNs in the BPT diagram

Investigating topics in galaxy formation and evolution requires a detailed understanding of the role of AGNs and it is critical to be able to reliably select or remove AGNs from samples of galaxies. If an AGN’s contributions are present but unrecognized, then measurements of its host’s properties such as star formation rate or gas-phase metallicity will be inaccurate. As a method for separating AGNs and star-forming galaxies, the BPT diagram is of particular interest because it uses commonly available optical emission line ratios and is already widely used in studies of narrow emission line galaxies. It is, however, not well understood what determines where an AGN lies on the BPT diagram. In this talk, we look at the line ratios of “pure” AGNs—BPT AGNs which have had their star formation contributions empirically removed. We account for this by matching AGNs to star-forming galaxies with similar host properties then subtract the emission fluxes from star formation. We find that such pure AGNs still populate the full range of the AGN branch, suggesting it is more than simply a mixing sequence between star formation and AGNs and that narrow-line regions have a wide variety of physical conditions. We explore how an AGN’s final position on the BPT diagram relates to host star formation, ionizing source properties, and narrow line region conditions. The methods employed here may be of use in uncovering the nature of AGNs at higher redshifts with Roman Space Telescope operations.