The objective of this chapter is to set the scene for the later chapters, in which the physics of many different aspects of active galactic nuclei and their interactions with their surroundings are studied in some detail. Much of the lore and terminology of the active galactic nuclei are the product of the historical development of the subject. The history of the discovery of different types of active galaxy and the techniques used to find them are described briefly in this chapter. A key feature of active galactic nuclei is that they are intrinsically broad-band, indeed multi-waveband, objects, each waveband providing complementary information, as well as possessing their own terminology and astrophysical infrastructure. There are several excellent books on different aspects of active galaxies. Active Galactic Nuclei by Robson (1999), An Introduction to Active Galactic Nuclei by Peterson (1997), Quasars and Active Galactic Nuclei – an Introduction by Kembhavi and Narlikar (1999), and Active Galactic Nuclei by Krolik (1999) can be recommended as providing a range of varied approaches to putting some order into their study.
Radio galaxies and high energy astrophysics
Cosmic rays, discovered by Hess in 1913, provided the first evidence for the existence of relativistic matter originating from extraterrestrial sources (Sect. 1.10). It was, however, only after the Second World War and the development of the new astronomies that the astrophysical role of high energy particles and cosmic magnetic fields could be addressed on the basis of astronomical observation.