and Cosmology

5.1 Introduction
179
Fig. 5.5. Radio maps at λ = 6 cm for two radio galaxies: the top
one is M84, an FR I radio source, the bottom one is 3C175, an
FR II source. The radiation from M84 in the radio is strongest
near the center and decreases outwards, whereas in 3C175
the most prominent components are the two radio lobes. The
radio lobe on the right is connected to the compact core by
a long and very thin jet, whereas on the opposite side no jet
(counter-jet) is visible
fects. Catalogs that are sampled at low frequencies will
predominantly select sources that have a steep spectrum,
i.e., in which the extended structures dominate,
whereas high-frequency samples will preferentially
contain core-dominated sources with a flat spectrum. 2
2 For this reason, radio surveys for gravitational lens systems, which
have been mentioned in Sect. 3.8.3, concentrate on sources with a flat
spectral index because these are dominated by the compact nucleus.
Multiple image systems are thus more easily recognized as such.
Fig. 5.6. The radio galaxy NGC 6251, with angular resolution
increasing towards the bottom. On large scales (and at low
frequencies), the two radio lobes dominate, while the core and
the jets are clearly prominent at higher frequencies. NGC 6251
has a counter-jet, but with significantly lower luminosity than
the main jet. Even at the highest resolution obtained by VLBI,
structure can still be seen. The jets have a very small opening
angle and are therefore strongly collimated
Synchrotron Radiation. Over a broad range in wavelengths,
the radio spectrum of AGNs follows a power
law of the form (5.2), with α ∼ 0.7 for the extended components
and α ∼ 0 for the compact core components.
Radiation in the radio is often linearly polarized, where
the extended radio source may reach a degree of polarization
up to 30% or even more. The spectral form and
the high degree of polarization are interpreted such that
the radio emission is produced by synchrotron radiation