High-Resolution Emission-Line Imaging of Seyfert Galaxies. II. Evidence for Anisotropic Ionizing Radiation
Abstract
In the preceding paper, we describe a direct imaging survey of Seyfert galaxies with "linear" radio structures and find that the major axes and spatial scales of the circumnuclear emission-line gas are very similar to those of the radio continuum sources. In the present paper, the nature of this close connection between thermal and relativistic gases is assessed in detail. Models in which the kinetic energy of the radio jets or plasmoids powers shock waves, which ionize the gas, seem energetically feasible but disagree with the off-nuclear line intensity ratios. Ionization by relativistic electrons is negligible, but they may contribute to the heating of the gas. We favor a scenario in which the radio jets and plasmoids shock, accelerate, and compress ambient and entrained gas, but the dominant source of ionization is the nonstellar nuclear ultraviolet continuum. This ultraviolet source appears to be partially beamed along the axis of the radio jet. Photoionization by ultraviolet synchrotron radiation generated via shocks in the ejecta may also contribute, especially in Seyfert 2 galaxies. A comparison between the number of ionizing photons, N_i_, inferred by extrapolation of the directly observed continuum, and the number of ionizing photons, N_Hβ_, required to generate the Hβ emission has been made for six galaxies in our sample. In at least two galaxies, we find N_i_ << N_Hβ_, suggesting that the gas is exposed to a higher ionizing flux than inferred from direct observations of the nucleus, and supporting the idea of partial beaming. Similarly, the energy in the continuum between 100 A and 1 micron, if emitted isotropically, is inadequate to fuel the thermal nuclear infrared sources, implying that the radiating dust is heated by a more luminous optical-ultraviolet source. We speculate that the nuclear infrared emission of Seyfert 2 galaxies arises from dust in molecular clouds exposed to the partially beamed radiation, and we predict that the 10 micron sources should align with the radio axes. The partial beaming of the optical-ultraviolet radiation may arise through "shadowing" by a dense, obscuring torus, or it could reflect the intrinsic distribution of photons radiated by the accretion disk as a function of angular distance from its rotation axis ("polar diagram").
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 1988
- DOI:
- 10.1086/166822
- Bibcode:
- 1988ApJ...334..121W
- Keywords:
-
- GALAXIES: JETS;
- GALAXIES: SEYFERT;
- GALAXIES: STRUCTURE;
- RADIO SOURCES: GENERAL