and Cosmology

6.3 X-Ray Radiation from Clusters of Galaxies
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Fig. 6.12. Left: the fraction of galaxies in the red distribution
(see Sect. 3.7.2) is shown as a function of Σ 5 ,an
estimator of the local galaxy number density based on
the projected distance of the fifth-nearest spectroscopically
confirmed neighbor galaxy within ±1000 km/s. Different
symbols correspond to different luminosity bins, as indicated.
Right: the same red fraction is plotted against a combination
of the local galaxy density Σ 5 and the luminosity of the galaxy
less luminous ones. Assuming that the mass-to-light ratio
does not vary substantially among cluster members,
this then indicates that the most massive galaxies have
smaller velocity dispersions. One way to achieve this
trend in the course of cluster evolution is by dynamical
interactions between cluster galaxies. Such interactions
tend to “thermalize” the velocity distribution of galaxies,
so that the mean kinetic energy of galaxies tends to
become similar. This then causes more massive galaxies
to become slower on average. If this interpretation
holds, then the morphology–density relation may be attributed
to these dynamical interactions, rather than to
the (so-called ram-pressure) stripping of the interstellar
medium as the galaxies move through the intracluster
medium.
E+A Galaxies. Galaxy clusters contain a class of galaxies
which is defined in terms of spectral properties.
These galaxies show strong Balmer line absorption in
their spectra, characteristic of A stars, but no [OII] or
Hα emission lines. The latter indicates that these galaxies
are not undergoing strong star formation at present,
whereas the former shows that there was an episode of
star formation within the past ∼ 1 Gyr, about as long
ago as the main-sequence lifetime of A stars. These
galaxies have been termed E+A galaxies since their
spectra appears like a superposition of that of A-stars
and that of otherwise normal elliptical galaxies. They
are interpreted as being post-starburst galaxies. Since
they were first seen in clusters, the interpretation of the
origin of E+A galaxies was originally centered on the
cluster environment – for example star-forming galaxies
falling into a cluster and having their interstellar
medium removed by tidal forces caused by the cluster
potential well and/or stripping as the galaxies move
through the intracluster medium. However, E+A galaxies
were later also found in different environments,
making the above interpretation largely obsolete. By investigating
the spatial correlation of these galaxies with
other galaxies shows that the phenomenon is not associated
with the large-scale environment. An overdensity
of neighboring galaxies can be seen only out to scales of
∼ 100 kpc. If the sudden turn-off of the star-formation
activity is indeed caused by an external perturbation,
it is therefore likely that it is caused by the dynamical
interaction of close neighboring galaxies. Indeed,
about 30% of E+A galaxies are found to have morphological
signatures of perturbations, such as tidal tails,
supporting the interaction hypothesis.
In fact, the spiral galaxies in clusters seem to differ
statistically from those of field spirals, in that the
fraction of disk galaxies with absorption-line spectra,
and thus no ongoing star formation, seems to be larger
in clusters than in the field by a factor ∼ 4, indicating