and Cosmology

5.5 Family Relations of AGNs
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Fig. 5.28. Sketch of our current understanding of the unification
of AGN types. The accretion disk is surrounded by
a thick torus containing dust which thus obscures the view to
the center of the AGN. When looking from a direction near
the plane of the disk, a direct view of the continuum source
and the BLR is blocked, whereas it is directly visible from
directions closer to the symmetry axis of the disk. The difference
between Seyfert 1 (and BLRG) and Seyfert 2 (and
NLRG) is therefore merely a matter of orientation relative to
the line-of-sight. If an AGN is seen exactly along the jet axis,
it appears as a blazar
Fig. 5.29. The contours show the color of the optical emission
in the Seyfert 2 galaxy NGC 1068, namely the flux ratio in
the U- and R-bands. The sticks indicate the strength and
orientation of the polarization in B-band light. The center of
the galaxy is located at Δα = 0 = Δδ. At its bluest (center
left), the polarization of the optical emission is strongest and is
perpendicular to the direction to the center of the galaxy; this
is the direction of polarization expected for local scattering by
electrons. Hence, where the scattering is strongest, the largest
fraction of direct light from the AGN is also observed, and the
optical spectrum of AGNs is considerably bluer than the stellar
light from galaxies
ties comparable to QSOs, the latter interpretation is
possible. In fact, distinguishing between the two possibilities
is not easy for individual ULIRGs, and in many
sources indicators of both strong star formation and
non-thermal emission (e.g., in the form of X-ray emission)
are found. This discovery indicates that in many