and Cosmology

6.3 X-Ray Radiation from Clusters of Galaxies
geneities on these length-scales. Thus, it may well be
that we live in a slightly overdense or underdense region
of the Universe, where the Hubble constant deviates
from the global value. In contrast to this, both the SZ
effect and the lensing method measure the Hubble constant
on truly cosmic scales, and both methods do so
in only a single step – there is no distance ladder involved.
For these reasons, these two methods are of great
importance in additionally confirming our H 0 measurements.
Another aspect adds to this, which must not
be underestimated: even if the same or a similar value
results from these measurements as the one from the
Hubble Key Project, we still have learned an important
fact, namely that the local Hubble constant agrees
with the one measured on cosmological scales – this
is one of the predictions of our cosmological model,
which can thus be tested in an impressive way. Indeed,
both methods have been applied to quite a number of
lens systems and luminous clusters showing an SZ effect,
respectively, and they yield values for H 0 which
are slightly smaller than, but compatible within the error
bars with the value of H 0 obtained from the Hubble
Key Project.
6.3.5 X-Ray Catalogs of Clusters
Originally, clusters of galaxies were selected by overdensities
of galaxies on the sphere using optical
methods. As we have seen, projection effects may play
a crucial role in this, in the form of coincidental overdensities
in the projected galaxy distribution, which do
not correspond to spatial overdensities. In addition, one
has the superposition of foreground and background
galaxies, which renders the selection more difficult the
farther the clusters are away from us.
A more reliable way of selecting clusters is by their
X-ray emission, since the hot X-ray gas signifies a deep
potential well, thus a real three-dimensional overdensity
of matter, so that projection effects become virtually
negligible. The X-ray emission is ∝ n 2 e , which again
renders projection effects improbable. In addition, the
X-ray emission, its temperature in particular, seems to
be a very good measure for the cluster mass, as we
will discuss further below. Whereas the selection of
clusters is not based on their temperature, but on the
X-ray luminosity, we shall see that L X is also a good
indicator for the mass of a cluster (see Sect. 6.4).
The first cosmologically interesting X-ray catalog of
galaxy clusters was the EMSS (Extended Medium Sensitivity
Survey) catalog. It was constructed from archival
images taken by the Einstein observatory which were
scrutinized for X-ray sources other than the primary
target in the field-of-view of the respective observation.
These were compiled and then further investigated using
optical methods, i.e., photometry and spectroscopy.
The EMSS catalog contains 835 sources, most of them
AGNs, but it also contains 104 clusters of galaxies.
Among these are six clusters at redshift ≥ 0.5; the most
distant is MS1054−03 at z = 0.83 (see Fig. 6.15). Since
the Einstein images all have different exposure times,
the EMSS is not a strictly flux-limited catalog. But with
the flux limit known for each exposure, the luminosity
function of clusters can be derived from this.
The same method as was used to compile the EMSS
was applied to ROSAT archival images by various
groups, leading to several catalogs of X-ray-selected
clusters. The selection criteria of these different groups,
and therefore of the different catalogs, differ. Since
ROSAT was more sensitive than the Einstein observatory,
these catalogs contain a larger number of clusters,
and also ones at higher redshift (Fig. 6.25). Furthermore,
ROSAT performed a survey of the full sky, the ROSAT
All Sky Survey (RASS). The RASS contains about 10 5
sources distributed over the whole sky. The identification
of extended sources in the RASS (in contrast to
non-extended sources – about five times more AGNs
than clusters are expected) yielded a catalog of clusters
as well which, owing to the relatively short exposure
times in the RASS, contains the brightest clusters. The
exposure time in the RASS is not uniform over the sky
since the applied observing strategy led to particularly
long exposures for the regions around the Northern and
Southern ecliptic pole (see Fig. 6.26).
One of the cluster catalogs that were extracted
from the RASS data is the HIFLUGCS
catalog. It consists of the 63 X-ray-brightest clusters
and is a strictly flux-limited survey, with
f X (0.1−2.4keV) ≥ 2.0 × 10 −11 erg s −1 cm −1 ; it excludes
the Galactic plane, |b|≥20 ◦ , as well as other
regions around the Magellanic clouds and the Virgo
Cluster of galaxies in order to avoid large column densities
of Galactic gas which lead to absorption, as well as
Galactic and other nearby X-ray sources. The extended
HIFLUGCS survey contains, in addition, several other
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