A comprehensive classification of galaxies in the SDSS: How to tell true from fake AGN?
R. Cid Fernandes (1), G. Stasinska (2), A. Mateus (1), N. Vale Asari, (1) (for the SEAGal collaboration) ((1) UFSC, Brazil, (2) LUTH, Observatoire, de Paris, France)

TL;DR
This paper introduces a new emission-line classification scheme for SDSS galaxies using the WHAN diagram, effectively distinguishing true AGN from retired galaxies and weak line galaxies, especially in cases with weak emission lines.
Contribution
The study develops a comprehensive galaxy classification method based on the WHAN diagram and a new criterion using the W_Ha equivalent width to differentiate true AGN from retired galaxies.
Findings
The bimodal distribution of W_Ha effectively separates active from retired galaxies.
Five distinct galaxy classes are identified within the WHAN diagram.
The classification scheme is validated through analysis of star formation histories.
Abstract
We use the W_Ha 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 (\xi) of the extinction-corrected L_Ha with respect to the Ha luminosity expected from photoionization by stellar populations older than 100 Myr. This ratio…
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