Unified theory of active galaxies

Any theory which seeks to explain how two or more types of active galaxy are actually the same class of object but simply observed at different orientations to the Earth. Some unification schemes are quite simple. For example a popular scheme seeks to link Seyfert type 1 and type 2 galaxies. The difference in these galaxies is the width of the spectral emission lines. The width of these lines indicate the speed at which the hydrogen clouds are orbiting the active galactic nucleus. In the unification theory the Seyfert galaxy's orientation to us determines whether observers on the Earth can see into the deep regions of the nucleus, where the faster moving clouds exist. If this region is blocked from our view then observers only see the slower clouds. Most unification schemes need to surround the active galactic nucleus with a torus of dusty material. It is the torus that blocks our view when the galaxy is orientated edge-on to us. Grand unification schemes attempt to link all active galaxies, such as Seyfert galaxies , quasars and BL Lacertae objects together. There is currently great debate amongst astronomers over whether radio loud active galaxies such as quasars and radio galaxies, can be unified with radio quiet active galaxies such as Seyferts.