The prime ingredients of active galactic nuclei

It is convenient to divide the necessary ingredients of active galactic nuclei into two types – the primary ingredients, which originate close to the black hole and its associated accretion disc, and secondary phenomena, which result from the interaction of the primary ingredients with the environment of the black hole. Figure 20.1 is a schematic diagram showing some of the components of typical models. The primary ingredients are intense non-thermal continuum radiation and fluxes of relativistic material in the form of highly collimated jets. The secondary phenomena result from the interaction of the primary components with the surrounding medium, in particular, gas clouds in the vicinity of the nucleus and the ambient interstellar and intergalactic gas. The former gives rise to the strong emission line spectrum observed at optical, ultraviolet and infrared wavelengths whilst the interactions of the relativistic jets with the interstellar and intergalactic gas give rise to the structures observed in extragalactic radio sources and in intense γ-ray emission. We study the physics of high energy particles in extragalactic radio sources and galactic nuclei and the role of relativistic beaming in the following chapters.

The continuum spectrum

As discussed in Chap. 18, active galactic nuclei contain intense continuum emission with non-thermal spectra. The examples illustrated in that chapter include typical spectra of Types 1 and 2 Seyfert galaxies (Fig. 18.5a and b), a composite quasar spectrum (Fig. 18.1) and a multi-waveband spectrum of the BL-Lac object OJ287 (Fig. 18.7).