and Cosmology

5. Active Galactic Nuclei
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The light of normal galaxies in the optical and near infrared
part of the spectrum is dominated by stars, with
small contributions by gas and dust. This is thermal radiation
since the emitting plasma in stellar atmospheres
is basically in thermodynamical equilibrium. To a first
approximation, the spectral properties of a star can be
described by a Planck spectrum whose temperature depends
on the stellar mass and the evolutionary state of
the star. As we have seen in Sect. 3.9, the spectrum of
galaxies can be described quite well as a superposition
of stellar spectra. The temperature of stars varies
over a relatively narrow range. Only few stars are found
with T 40 000 K, and those with T 3000 K hardly
contribute to the spectrum of a galaxy, due to their
low luminosity. Therefore, as a rough approximation,
the light distribution of a galaxy can be described by
a superposition of Planck spectra from a temperature
range that covers about one decade. Since the Planck
spectrum has a very narrow energy distribution around
its maximum at h P ν ∼ 3k B T, the spectrum of a galaxy
is basically confined to a range between ∼ 4000 Å
and ∼ 20 000 Å. If the galaxy is actively forming stars,
young hot stars extend this frequency range to higher
frequency, and the thermal radiation from dust, heated
by these new-born stars, extends the emission to the
far-infrared.
However, there are galaxies which have a much
broader energy distribution. Some of these show significant
emission in the full range from radio wavelengths
to the X-ray and even Gamma range (see Fig. 3.3). This
emission originates mainly from a very small central region
of such an active galaxy which is called the active
galactic nucleus (AGN). Active galaxies form a family
of many different types of AGN which differ in their
spectral properties, their luminosities and their ratio of
nuclear luminosity to that of the stellar light. The optical
spectra of three AGNs are presented in Fig. 5.1.
Some classes of AGNs, in particular the quasars,
belong to the most luminous sources in the Universe,
and they have been observed out to the highest measured
redshifts (z ∼ 6). The luminosity of quasars can
exceed the luminosity of normal galaxies by a factor
of a thousand. This luminosity originates from a very
small region in space, r ≤ 1 pc. The optical/UV spectra
Fig. 5.1. Optical spectra of three AGNs. The top panel displays
the spectrum of a quasar at redshift z ∼ 2, which shows the
characteristic broad emission lines. The strongest are Lyα of
hydrogen, and the CIV-line and CIII]-line of triple and double
ionized carbon, respectively (where the squared bracket means
that this is a semi-forbidden transition, as will be explained in
Sect. 5.4.2). The middle panel shows the spectrum of a nearby
Seyfert galaxy of Type 1. Here both very broad emission
lines and narrow lines, in particular of double ionized oxygen,
are visible. In contrast, the spectrum in the bottom panel,
of a Seyfert galaxy of Type 2, shows only relatively narrow
emission lines
Peter Schneider, Active Galactic Nuclei.
In: Peter Schneider, Extragalactic Astronomy and Cosmology. pp. 175–222 (2006)
DOI: 10.1007/11614371_5 © Springer-Verlag Berlin Heidelberg 2006