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Semi-empirical analysis of Sloan Digital Sky Survey galaxies - III. How to distinguish AGN hosts

Abstract : This paper considers the techniques to distinguish normal star-forming (NSF) galaxies and active galactic nuclei (AGNs) hosts using optical spectra. The observational data base is a set of 20000 galaxies extracted from the Sloan Digital Sky Survey, for which we have determined the emission-line intensities after subtracting the stellar continuum obtained from spectral synthesis. Our analysis is based on photoionization models computed using the stellar ionizing radiation predicted by population synthesis codes (essentially STARBURST99) and, for the AGNs, a broken power-law spectrum. We explain why, among the four classical emission-line diagnostic diagrams, ([OIII]/Hbeta versus [OII]/Hbeta, [OIII]/Hbeta versus [NII]/Halpha (the BPT diagram), [OIII]/Hbeta versus [SII]/Halpha and [OIII]/Hbeta versus [OI]/Halpha), the BPT one works best. We show, however, that none of these diagrams is efficient in detecting AGNs in metal-poor galaxies, should such cases exist. We propose a new divisory line between `pure' NSF galaxies and AGN hosts: y = (-30.787 + 1.1358x + 0.27297x2) tanh (5.7409x) - 31.093, where y = log([OIII]/Hbeta), and x = log([NII]/Halpha). According to our models, the divisory line drawn empirically by Kauffmann et al. includes among NSF galaxies objects that may have an AGN contribution to Hbeta of up to 3 per cent. The Kewley et al. line allows for an AGN contribution of roughly 20 per cent. About 20 per cent of the galaxies in our entire sample that can be represented in the BTP diagram are found between our divisory line and the Kauffmann et al. line, meaning that the local Universe contains a fair proportion of galaxies with a very low-level nuclear activity, in agreement with the statistics from observations of nuclei of nearby galaxies. We also show that a classification into NSF and AGN galaxies using only [NII]/Halpha is feasible and useful.
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Grazyna Stasinska, Roberto Cid Fernandes, Abílio Mateus, Laerte, Jr. Sodré, Natalia V. Asari. Semi-empirical analysis of Sloan Digital Sky Survey galaxies - III. How to distinguish AGN hosts. Monthly Notices of the Royal Astronomical Society, 2006, 371, pp.972-982. ⟨10.1111/j.1365-2966.2006.10732.x⟩. ⟨hal-03796875⟩

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