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 (AGNs) 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 STARBURST99) and, for the AGNs, a broken power-law spectrum. We explain why, among the four classical emission-line diagnostic diagrams, ([O III]/H beta versus [O II]/H beta, [O III]/H beta versus [N II]/H alpha (the BPT diagram), [O III]/H beta versus [S III]/H alpha and [O III]/H beta versus [O III]/H alpha), the BPT one works best. We show, however, that none of these diagrams is efficient in detecting AGNs in metal-poor galaxies, should such cases exist. We propose a new divisory line between 'pure' NSF galaxies and AGN hosts: y = (-30.787 + 1.1358x + 0.27297x(2)) tanh (5.7409x) - 31.093, where y= log([O III]/H beta), and x = log([N II]/H alpha). According to our models, the divisory line drawn empirically by Kauffmann et al. includes among NSF galaxies objects that may have an AGN contribution to H beta of up to 3 per cent. The Kewley et al. line allows for an AGN contribution of roughly 20 per cent. About 20 per cent of the galaxies in our entire sample that can be represented in the BTP diagram are found between our divisory line and the Kauffmann et al. line, meaning that the local Universe contains a fair proportion of galaxies with a very low-level nuclear activity, in agreement with the statistics from observations of nuclei of nearby galaxies. We also show that a classification into NSF and AGN galaxies using only [N II]/H alpha is feasible and useful. Finally, we propose a new classification diagram, the DEW diagram, plotting D-n(4000) versus max(EW[O II], EW[Ne III]). This diagram can be used with optical spectra for galaxies with redshifts up to z = 1.3, meaning an important progress over classifications proposed up to now. Since the DEW diagram requires only a small range in wavelength, it can also be used at even larger redshifts in suitable atmospheric windows. It also has the advantage of not requiring stellar synthesis analysis to subtract the stars and of allowing one to see all the galaxies in the same diagram, including passive galaxies.