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Abstract: In the unification scheme of Seyfert galaxies, a dusty torus blocks thecontinuum source and broad line region in Seyfert 2 galaxies. However it is notclear whether or not and to what extent the torus affects the narrow linespectra. In this paper, we show that Seyfert 1 and Seyfert 2 galaxies havedifferent distributions on the OIII-H$\beta $ vs NII-H$\alpha$ diagram BPTdiagram for narrow lines. Seyfert 2 galaxies display a clear left boundary onthe BPT diagram and only 7.3% of them lie on the left. By contrast, Seyfert 1galaxies do not show such a cutoff and 33.0% of them stand on the left side ofthe boundary. Among Seyfert 1 galaxies, the distribution varies with theextinction to broad lines. As the extinction increases, the distribution on BPTdiagram moves to larger NII-H$\alpha$ value. We interpret this as an evidencefor the obscuration of inner dense narrow line region by the dusty torus. Wealso demonstrate that the OIII and broad line luminosity correlation dependson the extinction of broad lines in the way that high extinction objects havelower uncorrected OIII luminosities, suggesting that OIII is partiallyobscured in these objects. Therefore, using OIII as an indicator for thenuclear luminosity will systematically under-estimate the nuclear luminosity ofSeyfert 2 galaxies.

Author: Kai Zhang, Tinggui Wang, Xiaobo Dong, Honglin Lu

Source: https://arxiv.org/

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