Direct Evidence for Anisotropy: Radio Maps and their Relation to Optical Morphology

Summary

Abstract

Double radio sources provide perhaps the most graphic evidence for anisotropy in AGN, and observations of their large-scale structure can potentially give us a direct means of measuring the emission axis of the AGN itself. Attempts to unify radio sources using orientation and doppler boosting arguments alone have been relatively successful, however recent discoveries of correlated asymmetries on the kpc- to Mpc-scale in the radio, optical and IR may pose serious problems for such models.

Introduction

At first sight a paper about the large-scale (kpc – Mpc) structure in radio galaxies appears to have little direct relevance to the nature of the compact object in AGN. If we are considering evidence for anisotropy however then radio sources provide the most graphic evidence for such emission from the AGN – for example 4C74.26 (Fig. 1) shows all the principal features: an essentially linear structure with a one-sided jet linking the core with the southern lobe, the alignment of core and hotspots and a one-sided VLBI jet aligned with the large-scale jet. Potentially, radio sources could provide us with one of the best ways of determining the orientation of the emission axis of the AGN, however the recent discovery of large-scale correlated asymmetries, may not only make this a difficult task, but may also provide strong evidence against unified models of AGN phenomena in terms of orientation dependencies.

So as to limit my discussion I will consider only high-power objects with P(178 MHz) ≥ 10^25.5 W Hz sr⁻¹.