6. CONCLUSIONS

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.