1.1 The AGN Zoo
The large number of classes and subclasses appearing in AGN literature could disorientate physicists or even astronomers working in other fields. A simplified classification, however, can be made based on only two parameters, that is radio-loudness and the width of the emission lines, as summarized in Table 1 (see, e.g., Urry and Padovani 1995).
Although it was the strong radio emission of some quasars that led to their discovery about 35 years ago, it soon became evident that the majority of quasars were actually radio-quiet, that is most of them were not detected by the radio telescopes of the time. It then turned out that radio-quiet did not mean radio-silent, that is even radio-quiet AGN can be detected in the radio band. Why then the distinction? If one plots radio luminosity versus optical luminosity for complete samples of optically selected sources, it looks like there are two populations, the radio quiet one having, for the same optical power, a radio power which is about 3-4 orders of magnitudes smaller. The distribution of the luminosity ratio Lr / Lopt for complete samples, including the upper limits on the radio luminosity, appears to be bimodal, with a dividing line at a value Lr / Lopt ~ 10 (e.g., Stocke et al. 1992; both luminosities are in units of power/Hz). It would therefore be better to call the two classes ``radio-strong'' and ``radio-weak'' but the original names are still used. Note that only about 10-15% of AGN are radio-loud.
The other main feature used in AGN classification is the width, in case they are present, or the absence, of emission lines. These are produced by the recombination of ions of various elements (most notably H, He, C, N, O, Ne, Mg, Fe). Their width is due to the Doppler effect thought to result from more or less ordered motion around the central object. AGN are then divided in Type 1 (broad-lined) and Type 2 (narrow-lined) objects according to their line-widths, with 1000 km/s (full width half maximum) being the dividing value. Some objects also exist with unusual emission line properties, such as BL Lacs, which have very weak emission lines with typical equivalent widths (a measure of the ratio between line and continuum luminosity) < 5 Å.
As illustrated in Table 1, we then have radio-quiet Type 2 and Type 1 AGN, that is Seyfert 2 galaxies and Seyfert 1 galaxies/radio-quiet quasars (QSO) respectively. Radio-loud Type 2 AGN are radio galaxies (sometimes also called narrow-line radio galaxies [NLRG] to distinguish them from the broad-lined ones), classified as Fanaroff-Riley (Fanaroff and Riley 1974) I and II (FR I and II) according to their radio morphology (connected with their radio power), while radio-loud Type 1 AGN are broad-line radio galaxies (BLRG) and radio quasars. Finally, radio-loud sources with very weak emission lines are known as BL Lacertae objects, from the name of the class prototype, which was originally presumed to be a variable star in the Lacerta constellation.
Concentrating on the radio-loud sources, to which most of this paper is devoted, the BLRG are, at least in my view, simply local versions of radio quasars where we can detect the host galaxy, as Seyfert 1 galaxies are local versions of QSO (the possible reasons why we do not see the high-redshift counterparts of Seyfert 2 galaxies are discussed by Padovani 1998). Radio quasars are generally divided into steep-spectrum radio quasars (SSRQ) and flat-spectrum radio quasars (FSRQ), according to the value of their radio spectral index at a few GHz (r = 0.5 being usually taken as the dividing line, with f -). This distinction reflects the size of the radio emitting region. In fact, radio emission in these sources is explained in terms of synchrotron radiation (that is radiation from relativistic particles moving in a magnetic field), which for extended regions has a relatively steep spectrum (r ~ 0.7). On the other hand, nuclear, compact emission has a flatter spectrum, thought to be the result of the superposition of various self-absorbed components. The flat radio spectrum then indicates that nuclear emission dominates over the more extended emission, generally associated with the so-called ``radio-lobes.'' In fact, flat-spectrum quasars are generally core-dominated in the radio band, that is emission from the core is much stronger than emission from the extended regions, unlike for example SSRQ or narrow-line radio galaxies which are both lobe-dominated. Note that even though FSRQ have strong broad lines they are also included in the ``Type 0'' column in Table 1 because their multifrequency spectra are dominated by non-thermal emission as in BL Lac objects.