We have reviewed a number of recent papers dealing with the most controversial aspects of AGNs. They suggest interesting conclusions:
- All Seyfert galaxies can be essentially described by a single parameter, the X-ray column density which steadily increases from Seyfert 1s to Seyfert 1.8s to Seyfert 1.9s to Seyfert 2s.
- AGNs belong to two main classes: the high-ionization AGNs (Seyfert 1s and 2s) and the low-ionization AGNs (Liners) which probably differ by the accretion rate onto the central BH, Liners having low accretion rates and consequently being powered by an unefficient advection dominated accretion flow.
- HPQs are high-ionization radio loud AGNs with a relativistic jet pointing in the direction of the observer, while BLLs are low-ionization AGNs. Many blazars have been classified as BLLs on the basis of unsufficient data; most objects with weak broad emission lines are HPQs.
- Radio loud AGNs may host a rapidly rotating BH, and radio quiet AGNs a slowly rotating BH. Rapidly spinning BHs could be the result of the merger of two similar mass BHs.
- Many early-type spirals have a nuclear emission line spectrum intermediate between Liners and starbursts (the so-called weak [O I]-Liners); they are probably old starbursts dominated by supernova remnants.
- Cooling flow clusters often show near their center a filamentary structure which has a Liner-like spectrum most probably ionized by stellar processes rather than by an AGN; on the other hand, the central galaxy in these clusters is often a FR I radio galaxy, i.e. a genuine AGN. The true nature of an emission line nebulosity observed in a FR I galaxy centrally located in a cooling flow cluster is therefore somewhat ambiguous.
- ULIGs are powered by starbursts induced by merging processes; many are weak [O I]-Liners. A significant fraction of all ULIGs contains an AGN which is probably the consequence of the merging; the AGN may, in some cases, be the major source of energy.
- Type 2 QSOs exist; they are in general quite inconspicuous as both the broad emission lines and the nuclear continuum are hidden from view. FR II radio galaxies with a high radio luminosity are type 2 QSOs.
Acknowledgement. We are pleased to thank Suzy Collin for constructive discussions and Areg Mickaelian for careful reading of the manuscript.
|ADAF||Advection Dominated Accretion Flow|
|AGN||Active Galactic Nuclei|
|BAL||Broad Absorption Line|
|BALQSO||Broad Absorption Line QSO|
|BLL||BL Lacertae object|
|BLR||broad line region|
|BLRG||Broad Line Radio Galaxy|
|BLS1||Broad Line Seyfert 1|
|FR I||Fanaroff-Riley type I|
|FR II||Fanaroff-Riley type II|
|FWHM||Full Width at Half Maximum|
|gE||giant Elliptical galaxy|
|HBL||High energy peaked BLL|
|HPQ||Highly Polarized Quasar|
|HST||Hubble Space Telescope|
|HX||Hard X-ray (2-10 keV)|
|ILR||Intermediate Line Region|
|IRAS||InfraRed Astronomical Satellite|
|LBL||Low energy peaked BLL|
|LERG||Low-Excitation Radio Galaxy|
|Liner||Low Ionization Nuclear Emission line Region|
|MB||Blue absolute magnitude|
|MR||Red absolute magnitude|
|NELG||Narrow Emission Line X-ray Galaxy (=NLXG)|
|NLR||Narrow Line Region|
|NLRG||Narrow Line Radio Galaxy|
|NLS1||Narrow Line Seyfert 1|
|NLXG||Narrow emission Line X-ray Galaxy|
|PAH||Polycyclic Aromatic Hydrocarbon|
|QSO||Quasi Stellar Object or quasar|
|R||Flux ratio of the core to the extended radio components|
|RS||Schwarzschild radius: 2GM / c2|
|RBL||Radio selected BLL|
|ROSAT||Röntgen Observatory SATellite|
|SMBBH||SuperMassive Binary Black Hole|
|S1h||Seyfert 1 galaxy with a hidden BLR|
|S1i||Seyfert 1 galaxy with an absorbed BLR visible in infrared|
|ULIG||UltraLuminous Infrared Galaxy|
|VBLR||Very Broad Line Region|
|XBL||X-ray selected BLL|