and Cosmology

5. Active Galactic Nuclei
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the redshift as cosmological redshift, 3C273 is located
at the large distance of D ∼ 500 h −1 Mpc. This huge distance
of the source then implies an absolute magnitude
of M B =−25.3 + 5 log h, i.e., it is about ∼ 100 times
brighter than normal (spiral) galaxies. Since the optical
source had not been resolved but appeared point-like,
this enormous luminosity must originate from a small
spatial region. With the improving determination of radio
source positions, many such quasars (quasi-stellar
radio sources = quasars) were identified in quick succession,
the redshifts of some being significantly higher
than that of 3C273.
5.1.2 Fundamental Properties of Quasars
In the following, we will review some of the most important
properties of quasars. Although quasars are not
the only class of AGNs, we will at first concentrate on
them because they incorporate most of the properties of
the other types of AGNs.
As already mentioned, quasars were discovered by
identifying radio sources with point-like optical sources.
Quasars emit at all wavelengths, from the radio to the
X-ray domain of the spectrum. The flux of the source
varies at nearly all frequencies, where the variability
time-scale differs among the objects and also depends
on the wavelength. In general, it is found that the variability
time-scale is smaller and its amplitude larger
when going to higher frequencies of the observed radiation.
The optical spectrum is very blue; most quasars
at redshifts z 2haveU − B < −0.3 (for comparison:
only hot white dwarfs have a similarly blue color index).
Besides this blue continuum, very broad emission
lines are characteristic of the optical spectrum. Some of
them correspond to transitions of very high ionization
energy (see Fig. 5.3).
The continuum spectrum of a quasar can often be
described, over a broad frequency range, by a power
law of the form
S ν ∝ ν −α , (5.2)
where α is the spectral index. α = 0 corresponds to
a flat spectrum, whereas α = 1 describes a spectrum in
which the same energy is emitted in every logarithmic
frequency interval. Finally, we shall point out again the
high redshift of many quasars.
5.1.3 Quasars as Radio Sources:
Synchrotron Radiation
The morphology of quasars in the radio regime depends
on the observed frequency and can often be very complex,
consisting of several extended source components
and one compact central one. In most cases, the extended
source is observed as a double source in the
form of two radio lobes situated more or less symmetrically
around the optical position of the quasar. These
lobes are frequently connected to the central core by
jets, which are thin emission structures probably related
to the energy transport from the core into the lobes. The
observed length-scales are often impressive, in that the
total extent of the radio source can reach values of up
to 1 Mpc. The position of the optical quasar coincides
with the compact radio source, which has an angular
extent of ≪ 1 ′′ and is in some cases not resolvable even
with VLBI methods. Thus the extent of these sources
is 1 mas, corresponding to r 1 pc. This dynamical
range in the extent of quasars is thus extremely large.
Classification of Radio Sources. Extended radio
sources are often divided into two classes. Fanaroff–
Riley Type I (FR I) are brightest close to the core, and the
surface brightness decreases outwards. They typically
have a luminosity of L ν (1.4 GHz) 10 32 erg s −1 Hz −1 .
In contrast, the surface brightness of Fanaroff–Riley
Type II sources (FR II) increases outwards, and their luminosity
is in general higher than that of FR I sources,
L ν (1.4 GHz) 10 32 erg s −1 Hz −1 . One example for
each of the two classes is shown in Fig. 5.5. FR II radio
sources often have jets; they are extended linear
structures that connect the compact core with a radio
lobe. Jets often show internal structure such as knots
and kinks. Their appearance indicates that they transport
energy from the core out into the radio lobe. One
of the most impressive examples of this is displayed in
Fig. 5.6.
The jets are not symmetric. Often only one jet is observed,
and in most sources where two jets are found
one of them (the “counter-jet”) is much weaker than
the other. The relative intensity of core, jet, and extended
components varies with frequency, for sources
as a whole and also within a source, because the components
have different spectral indices. For this reason,
radio catalogs of AGNs suffer from strong selection ef-