and Cosmology

3.1 Classification
these names are only historical and are not meant to
describe an evolutionary track!
Obviously, the morphological classification is at least
partially affected by projection effects. If, for instance,
the spatial shape of an elliptical galaxy is a triaxial
ellipsoid, then the observed ellipticity ɛ will depend on
its orientation with respect to the line-of-sight. Also,
it will be difficult to identify a bar in a spiral that is
observed from its side (“edge-on”).
Besides the aforementioned main types of galaxy
morphologies, others exist which do not fit into the
Hubble scheme. Many of these are presumably caused
by interaction between galaxies (see below). Furthermore,
we observe galaxies with radiation characteristics
that differ significantly from the spectral behavior of
“normal” galaxies. These galaxies will be discussed
next.
3.1.2 Other Types of Galaxies
The light from “normal” galaxies is emitted mainly by
stars. Therefore, the spectral distribution of the radiation
from such galaxies is in principle a superposition of
the spectra of their stellar population. The spectrum of
stars is, to a first approximation, described by a Planck
function (see Appendix A) that depends only on the
star’s surface temperature. A typical stellar population
covers a temperature range from a few thousand Kelvin
up to a few tens of thousand Kelvin. Since the Planck
function has a well-localized maximum and from there
steeply declines to both sides, most of the energy of
such “normal” galaxies is emitted in a relatively narrow
frequency interval that is located in the optical and NIR
sections of the spectrum.
In addition to these, other galaxies exist whose spectral
distribution cannot be described by a superposition
of stellar spectra. One example is the class of active
galaxies which generate a significant fraction of their
luminosity from gravitational energy that is released in
the infall of matter onto a supermassive black hole, as
was mentioned in Sect. 1.2.4. The activity of such objects
can be recognized in various ways. For example,
some of them are very luminous in the radio and/or
in the X-ray portion of the spectrum (see Fig. 3.3), or
they show strong emission lines with a width of several
thousand km/s if the line width is interpreted as due
to Doppler broadening, i.e., Δλ/λ = Δv/c. Inmany
cases, by far the largest fraction of luminosity is produced
in a very small central region: the active galactic
nucleus (AGN) that gave this class of galaxies its name.
In quasars, the central luminosity can be of the order of
∼ 10 13 L ⊙ , about a thousand times as luminous as the total
luminosity of our Milky Way. We will discuss active
galaxies, their phenomena, and their physical properties
in detail in Chap. 5.
Another type of galaxy also has spectral properties
that differ significantly from those of “normal” galaxies,
namely the starburst galaxies. Normal spiral galaxies
like our Milky Way form new stars at a star-formation
rate of ∼ 3M ⊙ /yr which can be derived, for instance,
from the Balmer lines of hydrogen generated in the
HII regions around young, hot stars. By contrast, elliptical
galaxies show only marginal star formation or
none at all. However, there are galaxies which have
a much higher star-formation rate, reaching values of
89
Fig. 3.3. The spectrum of a quasar (3C273)
in comparison to that of an elliptical galaxy.
While the radiation from the elliptical
is concentrated in a narrow range spanning
less than two decades in frequency,
the emission from the quasar is observed
over the full range of the electromagnetic
spectrum, and the energy per logarithmic
frequency interval is roughly constant. This
demonstrates that the light from the quasar
cannot be interpreted as a superposition of
stellar spectra, but instead has to be generated
by completely different sources and by
different radiation mechanisms