Introduction

In the earlier chapters of this book we have covered the present-day understanding of quasars and AGN, based on certain paradigms. We may refocus on these to begin with.

• Quasars and AGN are extragalactic phenomena that form natural steps in the overall scenario of galactic evolution.

• The redshifts of these objects are of cosmological origin.

• The primary source of production of the energy emitted by these objects is the gravity of a highly collapsed supermassive object, which is idealized as a spinning black hole with an accretion disk.

• The ejection of matter from the central region (in the form of jets) is relativistic.

• Relativistic beaming and orientation effects play a key role in explaining certain crucial observed features of these objects.

Astronomy has developed through paradigms, some of which were correct right from the beginning whereas others have had to be corrected, modified or abandoned. From the early days of the geocentric theory to the colliding galaxies hypothesis of radio sources, astronomy also has a history in which a majority have enthusiastically subscribed to a mistaken paradigm, which has thus become dogma. It is against this background that we now look at the above assumptions with the eyes of a sceptic.

Quasars, AGN and galaxies

Unlike stellar evolution, astronomers are still far from piecing together a scenario of galactic evolution in which all the different types of galaxy would find a natural place.