and Cosmology

5. Active Galactic Nuclei
210
Fig. 5.30. The elliptical
galaxy NGC 4261. The
left-hand panel shows
an optical image of this
galaxy together with the
radio emission (shown in
orange). An HST image
showing the innermost
region of the galaxy is
shown on the right. The
jet is virtually perpendicular
to the central disk
of gas and dust, which
is in agreement with the
theoretical picture in the
context of a unification
model
objects, the processes of strong star formation and accretion
onto a SMBH are linked. For both processes, large
amounts of gas are necessary, and the fact that both starburst
galaxies and AGNs are often found in interacting
galaxies, where the disturbance in the gravitational field
provides the conditions for a gas flow into the center of
the galaxy, suggests a link between the two phenomena.
Next we will examine how blazars fit into this unified
scheme. A first clue comes from the fact that all
blazars are radio sources. Furthermore, in our interpretation
of superluminal motion (Sect. 5.3.3) we saw
that the appearance and apparent velocity of the central
source components depend on the orientation of the
source with respect to us, and that it requires relativistic
velocities of the source components. To obtain an interpretation
of the blazar phenomenon that fits into the
above scheme, we first need to discuss an effect that
results from Special Relativity.
5.5.2 Beaming
Due to relativistic motion of the source components
relative to us, another effect occurs, known as beaming.
Due to beaming, the relation between source luminosity
and observed flux from a moving source depends on its
velocity with respect to the observer. One aspect of this
phenomenon is the Doppler shift in frequency space:
the measured flux at a given frequency is different from
that of a non-moving source because the measured frequency
corresponds to a Doppler-shifted frequency in
the rest-frame of the source. Another effect described by
Special Relativity is that a moving source which emits
isotropically in its rest-frame has an anisotropic emission
pattern, with the angular distribution depending on
its velocity. The radiation is emitted preferentially in the
direction of the velocity vector of the source (thus, in the
forward direction), so that a source will appear brighter
if it is moving towards the observer. In Sect. 4.3.2, we
already mentioned the relation (4.44) between the radiation
intensity in the rest-frame of a source and in the
system of the observer. Due to the strong Doppler shift,
this implies that a source moving towards us appears
brighter by a factor
(
D + =
1
γ(1 − β cos φ)
) 2+α
(5.31)
than the source at rest, where α is the spectral index. Furthermore,
β = v/c, φ is the angle between the velocity
vector of the source component and the line-of-sight
to the source, and the Lorentz factor γ = (1 − β 2 ) −1/2
has already been defined in Sect. 5.3.3. Even at weakly