Abstract
Active galactic nuclei were discovered in the 1940s, first by radio astronomy, using Grote Reber's radio sources, and then by optics, using Seyfert's galaxies. Carl Seyfert discovered very broad lines in the spectra of certain galaxies, corresponding to 10,000 km/s, with very compact, unresolved emission (region smaller than 10 parsecs). Only permitted lines (e.g. Balmer lines) show these broad line regions (BLRs). Forbidden lines ([OII], [OIII]...) are only a few hundred km/s long, and are confined to narrow line regions (NLR, Narrow Line Region). At the same time, very bright, point-like objects called quasars (quasi-stars) were being discovered. In 1963, Maarten Schmidt discovered that the spectrum of quasars could be interpreted as that of a galaxy, but at a very highredshift. They are therefore extremely luminous point-like objects, 1,000 times more luminous than the entire galaxy combined, with its 200 billion stars! Where does this energy come from? It can only come from the gravitational energy of matter falling on a supermassive black hole. Indeed, the efficiency with which this energy is released is far greater than the nuclear energy of fusion in stars: the latter recovers at best only 0.7% of the mc2 mass energy, whereas black holes can recover 10% on average. The central monster is a Schwarzschild (non-rotating) or Kerr (rotating) black hole, and the last stable orbits can be traced in both cases, just beyond the horizon, beyond which nothing leaves, not even light. For a rotating black hole, it is possible to extract even more energy, and in particular to eject some of the matter at relativistic velocity in the direction perpendicular to the accretion disk: these are the radio jets. The consequences and observational diagnostics are described in detail.
