A Black Hole and a Starburst Walk into A Bar: Unmixing and Diluting Emission Line Cocktails Served in the Local Universe

Standard emission line diagnostics are able to segregate star-forming galaxies and Seyfert nuclei, and it is often assumed that ambiguous emission-line galaxies falling between these two populations are "composite" objects exhibiting both types of photoionization. We have developed a method that predicts the most probable H II and AGN components that could plausibly explain the "composite" classed objects solely on the basis of a single-aperture optical spectrum. The majority of our analysis is driven by empirical relationships revealed by SDSS data rather than theoretical models founded in assumptions. To verify our method, we have compared the predictions of our model with publicly released IFU data from the S7 survey and find that composite objects are not in fact a simple linear combination of the two types of emission. Further exploration reveals that the AGN component comprises clouds in the NLR of varying distances from the central ionizing source. Accounting for this e.ect in our decomposition method allows us to fully predict truly composite IFU flux ratios from a single aperture observation. Some objects, however, appear to be at variance with the predicted results, suggesting they may not be powered by black hole accretion and are therefore not starburst-Seyfert composites.