High-ionization nuclear emission-line region of Seyfert galaxies

Tohru Nagao, Yoshiaki Taniguchi, Takashi Murayama

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47 Citations (Scopus)


Recently, Murayama & Taniguchi proposed that a significant part of the high-ionization nuclear emission-line region (HINER) in Seyfert nuclei arises from the inner wall of dusty tori because type 1 Seyfert nuclei (S1s) show excess HINER emission with respect to type 2 Seyfert nuclei (S2s). This means that the radiation from the HINER is anisotropic, and thus statistical properties of the HINER emission can be used to investigate the viewing angle toward dusty tori for various types of Seyfert nuclei. In order to investigate viewing angles toward narrow-line Seyfert 1 galaxies (NLS1s) and intermediate types of Seyfert galaxies (i.e., type 1.5, 1.8, and 1.9 Seyfert galaxies, hereafter S1.5, S1.8, and S1.9, respectively), we apply this HINER test to them. We also apply the same test for S2s with and without the hidden broad-line region. The sample of Seyfert nuclei analyzed here consists of 124 Seyfert nuclei compiled from the literature. Our main results and suggestions are as follows: (1) The NLS1s are viewed more face-on toward dusty tori than the S2s. However, the HINER properties of the NLS1s are indistinguishable from those of the S1s. (2) The S1.5s appear to compose heterogeneous populations; e.g., (a) some of them may be seen from an intermediate viewing angle between S1s and S2s, (b) some S1.5s are basically Sis but a significant part of the broad-line region (BLR) emission is accidentally obscured by dense, clumpy gas clouds, or (c) some S1.5s are basically S2s, but a part of the BLR emission can be seen from some optically thin regions of the dusty torus. (3) The S1.8s, the S1.9s, and the objects showing either a broad Paβ line or polarized broad Balmer lines are seen from a large inclination angle, and the emission from the BLRs of such objects reaches us through optically thin parts of dusty tori. These three results support strongly the current unified model of Seyfert nuclei. Finally, (4) the line ratios of [Fe x] λ6374 to the low-ionization emission lines are rather more isotropic than those of [Fe VII] λ6087. Therefore, it is suggested that the [Fe x] λ6374 emission is not useful for investigating the viewing angle toward the dusty torus in Seyfert nuclei. The most plausible reason seems to be that the [Fe x] λ6374 emission is spatially extended, and thus its strength tends to show less viewing-angle dependence.

Original languageEnglish
Pages (from-to)2605-2628
Number of pages24
JournalAstronomical Journal
Issue number6
Publication statusPublished - 2000 Jun


  • Galaxies: Seyfert
  • Galaxies: nuclei
  • Quasars: emission lines


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