ABSTRACT
This chapter focuses on active galaxies that exhibit strong radio-frequency emission. In the centers of some galaxies, highly energetic processes, completely unrelated to stars, occur and produce an assortment of observable phenomena. The huge intensities of the radio emission from radio galaxies lead to absurdly large brightness temperatures, implying that this radiation is produced by non-thermal processes. The Milky Way is not an active galaxy, but following the idea that all large galaxies are active sometime in their history, we expect the center of the Milky Way to harbor the skeletal structure of the Active galactic nuclei (AGN) engine. AGN were originally discovered at visible wavelengths in the 1940s, when Carl Seyfert, Carl noticed that some spiral galaxies had especially bright nuclei. The term quasar can be understood from the history of their discovery. This chapter provides some of the analytical tools that one can use to infer the physical conditions in radio galaxies and quasars.
