Emission-line properties of Seyfert 2 nuclei |
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| Authors: | Q Gu J Melnick R Cid Fernandes D Kunth E Terlevich † R Terlevich † |
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| Institution: | Department of Astronomy, Nanjing University, Nanjing 210093, China;European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile;Departamento de Física - CFM - Universidade Federal de Santa Catarina, PO Box 476, Florianópolis 88040-900, SC, Brazil;Institut d'Astrophysique de Paris, 98bis Boulevard Arago, 75014 Paris, France;Instituto Nacional de Astrofísica, Optica y Electrónica, AP 51 y 216, 72000, Puebla, México |
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| Abstract: | This is the third paper of a series devoted to the study of the global properties of Joguet's sample of 79 nearby galaxies observable from the southern hemisphere, of which 65 are Seyfert 2 galaxies. We use the population synthesis models of Paper II to derive 'pure' emission-line spectra for the Seyfert 2 galaxies in the sample, and thus explore the statistical properties of the nuclear nebular components and their relation to the stellar populations. We find that the emission-line clouds suffer substantially more extinction than the starlight, and we confirm the correlations between stellar and nebular velocity dispersions and between emission-line luminosity and velocity dispersions, although with substantial scatter. Nuclear luminosities correlate with stellar velocity dispersions, but Seyferts with conspicuous star-forming activity deviate systematically towards higher luminosities. Removing the contribution of young stars to the optical continuum produces a tighter and steeper relation, L ∝σ4★ , consistent with the Faber–Jackson law.
Emission-line ratios indicative of the gas excitation such as O iii ]/Hβ and O iii ]/O ii ] are statistically smaller for Seyferts with significant star formation, implying that ionization by massive stars is responsible for a substantial and sometimes even a dominant fraction of the Hβ and O ii ] fluxes. We use our models to constrain the maximum fraction of the ionizing power that can be generated by a hidden active galactic nucleus (AGN). We correlate this fraction with classical indicators of AGN photoionization (i.e. X-ray luminosity and nebular excitation), but find no significant correlations. Thus, while there is a strong contribution of starbursts to the excitation of the nuclear nebular emission in low-luminosity Seyferts, the contribution of the hidden AGN remains elusive even in hard X-rays. |
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| Keywords: | galaxies: active galaxies: Seyfert galaxies: starburst galaxies: statistics |
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