Astronomy and Astrophysics – Astrophysics
Scientific paper
2005-10-26
Mon.Not.Roy.Astron.Soc.366:480-490,2006
Astronomy and Astrophysics
Astrophysics
12 pages, 13 figures. Final version accepted for publication in MNRAS
Scientific paper
10.1111/j.1365-2966.2005.09872.x
We present a study of the global properties of Joguet's sample of 65 Seyfert 2 galaxies. We use the population synthesis models of our Paper II to derive `pure' emission-line spectra, 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 confirm the correlations between stellar and nebular velocity dispersions and between emission line luminosity and velocity dispersions. Nuclear luminosities correlate with stellar velocity dispersions. Removing the contribution of young stars to the optical continuum produces a tighter and steeper relation, $L \propto \sigma_\star^4$, consistent with the Faber-Jackson law. Emission line ratios indicative of the gas excitation such as [OIII]/H$\beta$ and [OIII]/[OII] 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 H$\beta$ and [OII] fluxes. We use our models to constrain the maximum fraction of the ionizing power that can be generated by a hidden AGN. We correlate this fraction with classical indicators of AGN photo-ionization: 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. (abridged)
Cid Fernandes Roberto
Gu Qiusheng
Kunth Daniel
Melnick Jorge
Terlevich Elena
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