and Cosmology

5. Active Galactic Nuclei
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emission lines are still broad compared to the typical
velocities in normal galaxies.
Redshift. Quasar surveys are always flux limited, i.e.,
one tries to find all quasars in a certain sky region with
a flux above a predefined threshold. Only with such
a selection criterion are the samples obtained of any statistical
value. In addition, the selection of sources may
include further criteria such as color, variability, radio
or X-ray flux. For instance, radio surveys are defined by
S ν > S lim at a specific wavelength. The optical identification
of such radio sources reveals that quasars have
a very broad redshift distribution. For decades, quasars
have been the only sources known at z > 3. Below we
will discuss different kinds of AGN surveys.
In the 1993 issue of the quasar catalog by Hewitt &
Burbidge, 7236 sources are listed. This catalog contains
a broad variety of different AGNs. Although it is statistically
not well-defined, this catalog provides a good
indication of the width of the redshift and brightness
distribution of AGNs (see Fig. 5.8).
The luminosity function of quasars extends over
a very large range in luminosity, nearly three orders
of magnitude in L. It is steep at its bright end and
has a significantly flatter slope at lower luminosities
(see Sect. 5.6.2). We can compare this to the luminosity
function of galaxies which is described by a Schechter
function (see Sect. 3.7). While the faint end of the distribution
is also described here by a relatively shallow
power law, the Schechter function decreases exponentially
for large L, whereas that of quasars decreases as
a power law. For this reason, one finds quasars whose
luminosity is much larger than the value of L where the
break in the luminosity function occurs.
5.2 AGN Zoology
Quasars are the most luminous members of the class
of AGNs. Seyfert galaxies are another type of AGN
and were mentioned previously. In fact, a wide range of
objects are subsumed under the name AGN, all of which
Fig. 5.8. The redshift (left) and brightness distribution (right)
of QSOs in the 1993 Hewitt & Burbidge catalog. These distributions
provide no proper statistical information, but they
clearly show the width of the distributions. The decrease in
abundances for z ≥ 2.3 is a selection effect: many QSO surveys
start with a color selection, typically U − B < −0.3. If
z ≥ 2.3, the strong Lyα emission line moves into the B-filter
and hence the quasar becomes redder in this color index and
drops out of the color selection