A comprehensive classification of galaxies in the SDSS: How to tell true from fake AGN?

By R. C. Fernandes, G. Stasińska, A. Mateus, N. V. A. Ufsc, Brazil, Luth, O. Paris, France.. 2010

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We use the W Hα versus [NII]/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 10 8 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 W Hα , we set the practical division between wAGN and RGs at W Hα = 3 A. Five classes of galaxies are identified within the WHAN diagram: (i) pure star-forming galaxies: log[N II]/Hα 3 A; (ii) strong AGN (i.e. Seyferts): log[NII]/Hα > -0.4 and W Hα > 6A; (iii) weak AGN: log[N n]/Hα > -0.4 and W Hα between 3 and 6 A; (iv) RGs (i.e. fake AGN): W Hα < 3 A; (v) passive galaxies (actually, lineless galaxies): W Hα and W [NII] < 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.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/51482a3b23db663629d619b3a1e847d421f0b803

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Citations in the corpus

Galaxy pairs in the Sloan Digital Sky Survey – VIII. The observational properties of post-merger galaxies (2013, background): “Due to the uncertain nature of LINERs (e.g. Stasinska et al. 2008; Cid Fernandes et al. 2011; Yan & Blanton 2012), it is preferable to restrict ourselves to Seyfert-like spectra and we therefore impose a S/N > 5 requirement on the BPT lines for this basic classification.”
A characteristic oxygen abundance gradient in galaxy disks unveiled with CALIFA (2013, methodology): “For example, Cid Fernandes et al. (2011) propose using EW(Hα) to distinguish between retired (non star-forming) galaxies, weak AGNs, and star-forming galaxies.”
MOSFIRE Spectroscopy of Quiescent Galaxies at 1.5 < z < 2.5. II. Star Formation Histories and Galaxy Quenching (2018, background): “In the local universe, quiescent galaxies commonly feature weak emission lines due to the ionization from old stellar populations (Cid Fernandes et al. 2011).”
On the fundamental dichotomy in the local radio-AGN population (2012, background): “…that a significant proportion of LINERs are not due to AGN activity, but rather the emission lines are photoionised by post-asymptotic giant branch (post-AGB) stars in old galaxies: such cases can be identified as having an equivalent width of the Hα line below 3Å (Cid Fernandes et al. 2011).”
SDSS IV MaNGA – spatially resolved diagnostic diagrams: a proof that many galaxies are LIERs (2016, background): “Moreover, Cid Fernandes et al. (2011) have shown that the emission line galaxy population in SDSS is clearly bimodal in EW(Hα), and that 3 Å is a useful empirical separation between the two populations.”
SDSS IV MaNGA – metallicity and nitrogen abundance gradients in local galaxies (2017, background): “Previous work has also demonstrated that there exists a tight relation between diagnostic line ratios and EW(Hα) in emission, in the sense that LIERs have EW(Hα) < 3 Å in emission (Cid Fernandes et al. 2011; Sánchez et al. 2015; Belfiore et al. 2016b).”
Near infrared spectroscopy and star-formation histories of 3 ≤ z ≤ 4 quiescent galaxies (2018, background): “In fact, given that our galaxies have (a priori) low sSFR, both emission lines may be significantly contaminated by energy sources other than star-formation; an AGN, or low-ionization emission from old/intermediate-age stars (see, e.g., Cid Fernandes et al. 2010, 2011 for Balmer lines, and Yan et al”
The nature of LINER galaxies: Ubiquitous hot old stars and rare accreting black holes (2013, background): “Inconsistencies were found between the AGN-ionisation hypothesis, and either predicted emission line strengths (Cid Fernandes et al. 2011) or the spatial distribution of LINER-like ionised regions in the galaxies (Sarzi et al. 2010; Yan & Blanton 2012), but neither were conclusive, because they…”