A comprehensive classification of galaxies in the Sloan Digital Sky Survey: how to tell true from fake AGN?

We use the W-H alpha versus [N ii]/H alpha (WHAN) diagram introduced by us in previous work to provide a comprehensive emission-line classification of Sloan Digital Sky Survey galaxies. This classification is able to cope with the large population of weak line galaxies that do not appear in traditional diagrams due to a lack of some of the diagnostic lines. A further advantage of the WHAN diagram is to allow the differentiation between two very distinct classes that overlap in the low-ionization nuclear emission-line region (LINER) region of traditional diagnostic diagrams. These are galaxies hosting a weakly active galactic nucleus (wAGN) and 'retired galaxies' (RGs), i.e. galaxies that have stopped forming stars and are ionized by their hot low-mass evolved stars. A useful criterion to distinguish true from fake AGN (i.e. the RGs) is the value of xi, which measures the ratio of the extinction-corrected H alpha luminosity with respect to the H alpha luminosity expected from photoionization by stellar populations older than 108 yr. We find that xi follows a markedly bimodal distribution, with a xi 1 population composed by systems undergoing star formation and/or nuclear activity, and a peak at xi similar to 1 corresponding to the prediction of the RG model. We base our classification scheme not on xi but on a more readily available and model-independent quantity which provides an excellent observational proxy for xi: the equivalent width of H alpha. Based on the bimodal distribution of W-H alpha, we set the practical division between wAGN and RGs at W-H alpha = 3 A. Five classes of galaxies are identified within the WHAN diagram: pure star-forming galaxies: and W-H alpha > 3 A; strong AGN (i.e. Seyferts): and W-H alpha > 6 A; weak AGN: and W-H alpha between 3 and 6 A; RGs (i.e. fake AGN): W-H alpha < 3 A; passive galaxies (actually, lineless galaxies): W-H alpha and W-[N ii] < 0.5 A. A comparative analysis of star formation histories and of other physical and observational properties in these different classes of galaxies corroborates our proposed differentiation between RGs and wAGN in the LINER-like family. This analysis also shows similarities between strong and weak AGN on the one hand, and retired and passive galaxies on the other.