and Cosmology

5.5 Family Relations of AGNs
ing t act ∼ 2 × 10 8 yr. Hence, the SMBH of a current
day massive galaxy was active during about 2% of its
lifetime.
5.5 Family Relations of AGNs
5.5.1 Unified Models
In Sect. 5.2, different types of AGNs were listed. We
saw that many of their properties are common to all
types, but also that there are considerable differences.
Why are some AGNs seen as broad-line radio galaxies,
others as BL Lac objects? The obvious question arises
as to whether the different classes of AGNs consist of
rather similar objects which differ in their appearance
due to geometric or light propagation effects, or whether
more fundamental differences exist. In this section we
will discuss differences and similarities of the various
classes of AGNs and show that they presumably all
derive from the same physical model.
Common Properties. Common to all AGNs is a SMBH
in the center of the host galaxy, the supposed central
engine, and also an accretion disk that is feeding
the black hole. This suggests that a classification can
be based on M • and the accretion rate ṁ, or perhaps
more relevantly the ratio ṁ/ṁ edd . M • defines the maximum
(isotropic) luminosity of the SMBH in terms
of the Eddington luminosity, and the ratio ṁ/ṁ edd
describes the accretion rate relative to its maximum
value. Furthermore, the observed properties, in particular
the seemingly smooth transition between the
different classes, suggest that radio-quiet quasars and
Seyfert 1 galaxies basically differ only in their central
luminosity. From this, we would then deduce that
they have a similar value of ṁ/ṁ edd but differ in M • .
An analogous argument may be valid for the transition
from BLRGs to radio-loud quasars.
The difference between these two classes may be
due to the nature of the host galaxy. Radio galaxies (and
maybe radio-loud quasars?) are situated in elliptical galaxies,
Seyfert nuclei (and maybe radio-quiet quasars?)
in spirals. A correlation between the luminosity of the
AGN and that of the host galaxy also seems to exist.
This is to be expected if the luminosity of the AGN is
strongly correlated with the respective Eddington luminosity,
because of the correlation between the SMBH
mass in normal galaxies and the properties of the galaxy
(Sect. 3.5.3). Another question is how to fit blazars
and Seyfert 2 galaxies into this scheme.
Anisotropic Emission. In the context of the SMBH plus
accretion disk model, another parameter exists that will
affect the observed characteristics of an AGN, namely
the angle between the rotation axis of the disk and the
direction from which we observe the AGN. We should
mention that in fact there are many indications that the
radiation of an AGN is not isotropic and thus its appearance
is dependent on this direction. Among these are the
observed ionization cones in the NLR (see Fig. 5.24)
and the morphology of the radio emission, as the radio
lobes define a preferred direction. Furthermore, our
discussion of superluminal motion has shown that the
observed superluminal velocities are possible only if the
direction of motion of the source component is close to
the direction of the line-of-sight. The X-ray spectrum
of many AGNs shows intrinsic (photoelectric) absorption
caused by high column density gas, where this
effect is mainly observed in Seyfert 2 galaxies. Because
of these clear indications it seems obvious to examine
the dependence of the appearance of an AGN on the
viewing direction. For example, the observed difference
between Seyfert 1 and Seyfert 2 galaxies may simply
be due to a different orientation of the AGN relative to
the line-of-sight.
Broad Emission Lines in Polarized Light. In fact,
another observation of anisotropic emission provides
a key to understanding the relation between AGN types,
which supports the above idea. The galaxy NGC 1068
has no visible broad emission lines and is therefore
classified as a Seyfert 2 galaxy. Indeed, it is considered
an archetype of this kind of AGN. However, the
optical spectrum of NGC 1068 in polarized light shows
broad emission lines (Fig. 5.27) such as one would find
in a Seyfert 1 galaxy. Obviously the galaxy must have
a BLR, but it is only visible in polarized light. The
photons that are emitted by the BLR are initially unpolarized.
Polarization may be induced through scattering
of the light, however, where the direction perpendicular
to the directions of incoming and scattered photons
define a preferred direction, which then defines the
polarization direction.
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