Difference in Narrow Emission Line Spectra of Seyfert 1 and 2 galaxies

TL;DR

This study reveals that Seyfert 1 and 2 galaxies exhibit distinct narrow emission line spectra, influenced by the dusty torus, affecting the interpretation of their nuclear luminosities and the obscuration of their narrow line regions.

Contribution

It provides evidence that the dusty torus impacts narrow line spectra and the [OIII] luminosity, offering new insights into Seyfert galaxy unification models.

Findings

Seyfert 2 galaxies have a distinct boundary on the BPT diagram not seen in Seyfert 1s.

The distribution of Seyfert 1 galaxies on the BPT diagram varies with broad line extinction.

[OIII] luminosity is partially obscured in high-extinction 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 paper, we show that Seyfert 1 and Seyfert 2 galaxies have different distributions on the [OIII]/H$\beta $ vs [NII]/H$\alpha$ 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 [NII]/H$\alpha$ value. We interpret this as an evidence for the obscuration of inner dense narrow line region by the dusty torus. We also demonstrate that the [OIII] and broad line luminosity correlation depends on the extinction of broad lines in the way that high extinction objects have lower uncorrected [OIII] luminosities, suggesting that [OIII] is partially obscured in these objects. Therefore, using [OIII] as an indicator for the nuclear luminosity will systematically under-estimate the nuclear luminosity of Seyfert 2 galaxies.