A comprehensive classification of galaxies in the SDSS: How to tell true from fake AGN?
Abstract
We use the WHa versus [NII]/Ha (WHAN) diagram to provide a comprehensive emission-line classification of SDSS 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 LINER region of traditional diagnostic diagrams. These are galaxies hosting a weakly active nucleus (wAGN) and "retired galaxies" (RGs), i.e. galaxies that have stopped forming stars and are ionized by their hot evolved low-mass stars. A useful criterion to distinguish true from fake AGN (i.e. the RGs) is the ratio () of the extinction-corrected LHa with respect to the Ha luminosity expected from photoionization by stellar populations older than 100 Myr. This ratio follows a markedly bimodal distribution, with a >> 1 population composed by systems undergoing star-formation and/or nuclear activity, and a peak at ~ 1 corresponding to the prediction of the RG model. We base our classification scheme on the equivalent width of Ha, an excellent observational proxy for . Based on the bimodal distribution of WHa, we set the division between wAGN and RGs at WHa = 3 A. Five classes of galaxies are identified within the WHAN diagram: (a) Pure star forming galaxies: log [NII]/Ha < -0.4 and WHa > 3 A. (b) Strong AGN (i.e., Seyferts): log [NII]/Ha > -0.4 and WHa > 6 A. (c) Weak AGN: log [NII]/Ha > -0.4 and WHa between 3 and 6 A. (d) RGs: WHa < 3 A. (e) Passive galaxies (actually, line-less galaxies): WHa and W[NII] < 0.5 A. A comparative analysis of star formation histories and of other properties in these different classes of galaxies corroborates our proposed differentiation between RGs and weak AGN in the LINER-like family. (Abridged)