Black Holes, Accretion Disks and Relativistic Jets in Active Galactic Nuclei

As the conference logo clearly portrays, galaxies have "radio-active" nuclei. Indeed it was the remarkable discovery, that distant galaxies are sometimes accompanied by prodigiously energetic radio sources, that led to the convening of the first Texas conference on relativistic astrophysics, in 1964, 1 just in time for the even more noteworthy discovery of quasars. We now know that most galaxies, including our own, have definite nuclei and exhibit non-stellar activity at some level. At its weakest, this takes the form of barely discernible spectral lines; at its most dramatic it involves emission throughout the electromagnetic spectrum with luminosity far in excess of that of the host galaxy, most of it originating from a region smaller than our solar system. It is my task to provide a general summary of what astrophysicists think is happening in active galactic nuclei (AGN, henceforth), emphasizing recent developments.