Evidence for Anisotropy and Unification

Summary

Abstract

It is probable that the nuclei of all active galaxies radiate anisotropically either due to intrinsic beaming and/or due to extrinsic causes such as shadowing. Observations of scattered light and fluorescently excited extended line emission can be used to map the radiation pattern. The best established of the AGN unification schemes appears to be that for powerful radio sources. High redshift radio galaxies reveal their quasar/blazar nuclei clearly in their rest-frame ultraviolet radiation. Studies of local objects show in detail the physical process which are operating in these distant sources.

Introduction

The concept of radiation anisotropy has been remarkably successful in allowing the unification of various classes of active galaxy and quasar. Whilst the possibility of explaining the differing appearance of all the classes of active extragalactic sources simply in terms of orientation effects seems unrealistic now, it does appear that broad subclasses can be collapsed in this way. Thus, for example, the three subgroups of Seyfert 1s and 2s, Fanaroff/Riley class one (FR I) radio galaxies and BL Lac objects, and powerful radio galaxies and quasars, can be understood as entities having similar central engines and nuclear environments, but whose orientation with respect to the observer is the dominant variable determining the detailed appearance.

Fundamental differences in the nature of the central energy source may well exist and, if so, will certainly break the unification at some level.