and Cosmology

6. Clusters and Groups of Galaxies
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Fig. 6.9. The principle of dynamical friction. The gravitational
field of a massive particle (here indicated by the large symbol)
accelerates the surrounding matter towards its track. Through
this, an overdensity establishes on the backward side of its
orbit, the gravitational force of which decelerates the particle
sive galaxies will experience the strongest dynamical
friction, so that they are subject to a significant deceleration
through which they move deeper into the potential
well. The most massive cluster galaxies should therefore
be concentrated around the cluster center, so that
a spatial separation of galaxy populations with respect
to their masses occurs (mass segregation). If dynamical
friction acts over a sufficiently long time, the massive
cluster galaxies in the center may merge into a single
one. This is one possible explanation for the formation
of cD galaxies.
Dynamical friction also plays an important role in
other dynamical processes in astrophysics. For example,
the Magellanic Clouds experience dynamical friction
on their orbit around the Milky Way and thereby lose
kinetic energy. Consequently, their orbit will become
smaller over the course of time and, in a distant future,
these two satellite galaxies will merge with our Galaxy.
In fact, dynamical friction is of vital importance in
galaxy merger processes which occur in the evolution
of the galaxy population, a subject we will return to in
Sect. 9.6.
6.2.7 Intergalactic Stars in Clusters of Galaxies
The space between the galaxies in a cluster is filled
with hot gas, as visible from X-ray observations. In recent
years, it has been found that besides hot gas there
are also stars in between the galaxies. The detection of
such an intergalactic stellar population comes as a surprise
at first sight, because our understanding of star
formation implies that they can only form in the dense
centers of molecular clouds. Hence, one expects that
stars cannot form in intergalactic space. This is not necessarily
implied by the presence of intergalactic stars,
however, since they can also be stripped from galaxies
in the course of gravitational interactions between
galaxies in the cluster, and so form an intergalactic population.
The fate of these stars is thus comparable to
that of the interstellar medium, which is metal-enriched
by the processes of stellar evolution in galaxies before
it is removed from these galaxies and becomes part of
the intergalactic medium in clusters; otherwise, the substantial
metallicity of the ICM could not be explained.
The observation of diffuse optical light in clusters
of galaxies and, related to this, the detection of the
intracluster stellar population, is extremely difficult. Although
first indications have already been found with
photographic plate measurements, the surface brightness
of this cluster component is so low that even
with CCD detectors the observation is extraordinarily
challenging. To quantify this, we note that the surface
brightness of this diffuse light component is about
30 mag arcsec −2 at a distance of several hundred kpc
from the cluster center. This value needs to be compared
with the brightness of the night sky, which is about
21 mag arcsec −2 in the V-band. One therefore needs to
correct for the effects of the night sky to better than
a tenth of a percent for the intergalactic stellar component
to become visible in a cluster. Furthermore, cluster
galaxies and objects in the foreground and background
need to be masked out in the images, in order to measure
the radial profile of this diffuse component. This is possible
only up to a certain limiting magnitude, of course,
up to which individual objects can be identified. The existence
of weaker sources has to be accounted for with
statistical methods, which in turn use the luminosity
function of galaxies.
The diffuse light component is best investigated
in a statistical superposition of the images of several
galaxy clusters. Statistical fluctuations in the sky background
and uncertainties in the flatfield 3 determination
3 The flatfield of an image (or, more precisely, of the system consisting
of telescope, filter, and detector) is defined as the image of a uniformly
illuminated field, so that in the ideal case each pixel of the detector
produces the same output signal. This is not the case in reality, however,
as the sensitivity differs for individual pixels. For this reason,
the flatfield measures the sensitivity distribution of the pixels, which
is then accounted for in the image analysis.