Introduction

In this contribution, we will take a broad view of active galactic nuclei (AGNs), but concentrating on emission-line phenonema. As a result, some interesting topics such as blazars and jets, connections with starbursts, and the environments of AGNs will receive little attention, although these are important topics and are discussed at length elsewhere.

It is useful to begin a discussion of AGNs with some history of the subject, partly because the history of how a scientific field is launched and develops over time is interesting and instructive, but primarily because it gives us some insight into the observed properties of AGNs, which is essential for distinguishing them from other objects in the sky. As we introduce the observational properties of these sources, at least the basis of the sometimes complicated taxonomy we use to describe active galaxies will become clear. An underlying theme throughout this discussion will be the principle of “AGN unification,” which posits that the diverse taxonomy of AGNs has more to do with observational circumstances, such as inclination and obscuration effects, than with intrinsic physical differences among various types of AGN. Considerable effort has been expended in attempts to explain the broadest range of phenomena with the most-limited range of physical mechanisms and structures.

The discovery and nature of active galaxies

The word “activity” in connection with phenomena in the nuclei of galaxies appears to have originated with Ambartsumian (1968).