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The basic idea behind Unified Models of Active Galaxies is that all of
the active galaxies which we observe have the same internal structure
in their nucleus. All the differences that we see are caused by
the angle of the system as seen in our direction.
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When we see the system edge-on, the central region (the black
hole, the central accretion disk, and much of the broadline region)
is obscured. However we do see the molecular torus, and JETS!
Objects: Seyfert 2s, Narrow Line Radio Galaxies
When we see the system at a 45 degree angle, we start to see the central region. We now see both broad emission lines as well as narrow emission lines, and some direct, VERY HOT emission from the central accretion disk. Objects: Seyfert 1s, Broad Line Radio Galaxies, Type 1 Quasars
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For the VERY special case that we are seeing the system right
down the center, we are looking right INTO the Jet! Because matter
in the jet is moving at velocities close to the speed of light, that
radiation has extremely high luminosities and can vary over
short timescales. This jet emission is so strong, we hardly see
the Broadline or Narrowline regions, so no emission lines.
Objects: BL Lacertae (also called Blazars).