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

Abstract

We use the WHα versus [N ii]/Hα (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 ξ, which measures the ratio of the extinction‐corrected Hα luminosity with respect to the Hα luminosity expected from photoionization by stellar populations older than 108 yr. We find that ξ follows a markedly bimodal distribution, with a ξ≫ 1 population composed by systems undergoing star formation and/or nuclear activity, and a peak at ξ∼ 1 corresponding to the prediction of the RG model. We base our classification scheme not on ξ but on a more readily available and model‐independent quantity which provides an excellent observational proxy for ξ: the equivalent width of Hα. Based on the bimodal distribution of WHα, we set the practical division between wAGN and RGs at WHα= 3 Å. Five classes of galaxies are identified within the WHAN diagram: pure star‐forming galaxies: and WHα > 3 Å; strong AGN (i.e. Seyferts): and WHα > 6 Å; weak AGN: and WHα between 3 and 6 Å; RGs (i.e. fake AGN): WHα < 3 Å; passive galaxies (actually, lineless galaxies): WHα and W[N ii] < 0.5 Å. 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.

NGC 6845: metallicity gradients and star formation in a complex compact group

“…This diagram was proposed by Cid Fernandes et al (2011), and it allows to classify galaxies by using the [NII]/Hα ratio versus the equivalent width of the Hα emission line (WHAN diagram). In this diagram it is possible to separate the different ionization mechanism, i.e., young stars, AGN activity and ionization produced by hot low-mass evolved stars.…”

Evidence of ongoing AGN-driven feedback in a quiescent post-starburst E+A galaxy

“…They propose post-AGB stars as the ionizing source in many objects classified as LINERs. In a related work, Cid Fernandes et al (2011) study such cases and provide an estimated upper limit to EW(Hα) of about 3Å for post-AGB systems. This is considerably smaller than the EW obtained for SDSS J132401.63+454620.6 (about 8Å).…”

“…An alternative scenario involves contribution from both AGN and shocked gas, or contributions from shocked gas at different velocities (Dopita et al 1997;Nagar, Falcke & Wilson 2005). An additional process that can produce LINER-like emission is photoionization by evolved post asymptotic giant branch stars (post AGB stars) during a short but very hot and energetic phase (Stasińska et al 2008;Annibali et al 2010;Cid Fernandes et al 2011;Yan & Blanton 2012;Singh et al 2013).…”

The SAMI Galaxy Survey: spatially resolving the environmental quenching of star formation in GAMA galaxies

“…This distinction separates those galaxies in the star-forming "main sequence" from those which appear to be quenched. Cid Fernandes et al (2011) recommend a 3Å cut in EWHα to separate passive and star-forming galaxies. However, this recommendation was based on the use of SDSS single-fibre spectroscopy and is thus sensitive to the distribution of star formation within the system.…”