Difference in Narrow-Emission-Line Spectra of Seyfert 1 and 2 Galaxies

Abstract

In the unification scheme of Seyfert galaxies, a dusty torus blocks the continuum source and broad-line region in Seyfert 2 galaxies. However it is not clear whether or not and to what extent the torus affects the narrow-line spectra. In this Letter, we show that Seyfert 1 and Seyfert 2 galaxies have different distributions on the [O III]/Hβ versus [N II]/Hα diagram (BPT diagram) for narrow lines. Seyfert 2 galaxies display a clear left boundary on the BPT diagram and only 7.3% of them lie on the left. By contrast, Seyfert 1 galaxies do not show such a cutoff and 33.0% of them stand on the left side of the boundary. Among Seyfert 1 galaxies, the distribution varies with the extinction to broad lines. As the extinction increases, the distribution on BPT diagram moves to larger [N II]/Hα values. We interpret this as evidence for the obscuration of an inner dense narrow-line region by the dusty torus. We also demonstrate that the [O III] and broad-line luminosity correlation depends on the extinction of broad lines in the way that high-extinction objects have lower uncorrected [O III] luminosities, suggesting that [O III] is partially obscured in these objects. Therefore, using [O III] as an indicator for the nuclear luminosity will systematically underestimate the nuclear luminosity of Seyfert 2 galaxies.