Abstract

Recent work has revealed that a number of Seyfert 2 galaxies exhibit conicallyshaped regions of gas apparently illuminated by a collimated, nuclear ionizing source. In this work, we test one model for this collimation, namely that the cones result from shadowing of a compact nuclear continuum source by a thick, dusty torus. From the emission-line ratios measured for gas within the cones, we have calculated the number of ionizing photons emitted by the compact nucleus. Then, on the assumption that the nuclear source radiates isotropically, we have found the power incident on the torus, which is expected to be reradiated in the infrared. Given the uncertainties in the calculation, and the fact that the torus may be somewhat anisotropic in the infrared, we find the observed IRAS luminosities are consistent with the torus model in 9 objects with sufficient data to perform the calculation.

Introduction

There is now considerable evidence that the ionizing photons escape from the nuclear source anisotropically in many Seyfert 2 galaxies (see Wilson 1992 for a recent review). For some objects, this anisotropy becomes evident in the elongated morphologies detected in images taken through narrow-band filters centred in high excitation lines (e.g. [OIII]λ5007) or through “ionization maps” (Pogge 1988a,b) – ratios between the continuum subtracted images in [OIII]λ5007 and in Hα+ [NII]λλ6548,6583.

In this work, we combine new with existing data for a sample of nine Seyfert 2 galaxies with known “ionization cones” in order to test whether the collimation is the result of shadowing of radiation from a small, isotropic, nuclear source by a thick dusty torus, as suggested in the “Unified Scheme” for Seyfert galaxies (Antonucci & Miller 1985; Krolik & Begelman 1986).