Understanding the Fanaroff-Riley radio galaxy classification

TL;DR

This paper reviews and develops a comprehensive understanding of the Fanaroff-Riley classification of radio galaxies, integrating morphological, dust, and orientation properties within a unified framework.

Contribution

It introduces a novel qualitative framework that combines dust properties and orientations to explain the two radio galaxy types.

Findings

Inclusion of dust properties in the classification scheme

Correlation between dust orientation and radio morphology

Development of a unified understanding of FR types

Abstract

The simple, yet profoundly far-reaching classification scheme based on extended radio morphologies of radio galaxies, the Fanaroff-Riley classification has been a cornerstone in our understanding of radio galaxies. Over the decades since the recognition that there are two basic types of radio galaxy morphologies there have been several findings in different wavebands that have reported properties on different scales. Although it is realized that there may be intrinsic as well external causes an overarching view of how we may understand the two morphological types is missing. With the radio power-absolute magnitude relation (the Owen-Ledlow diagram) as backdrop we review and develop an understanding of the two radio galaxy types in the light of what is known about them. We have for the first time included the dust properties of the two FR classes together with the relative orientations of dust, host major axis and the radio axis to present a qualitative framework within which to understand the conditions under which they form. (Abridged).