Semi-empirical analysis of Sloan Digital Sky Survey galaxies - III. How to distinguish AGN hosts

This paper considers the techniques to distinguish normal star forming (NSF) galaxies and active galactic nuclei (AGN) hosts using optical spectra. The observational data base is a set of 20 000 galaxies extracted from the Sloan Digital Sky Survey, for which we have determined the emission line intensities after subtracting the stellar continuum obtained from spectral synthesis. Our analysis is based on photoionization models computed using the stellar ionizing radiation predicted by population synthesis codes (essentially Starburst 99) and, for the AGNs, a broken power-law spectrum. We explain why, among the four classical emission line