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*Astron. Astrophys. 339, 623-628 (1998)*
## 5. Conclusions
Using the Hubble constant measurements based on SZ and X-ray, and
the gas mass fraction measurements, we have suggested a possible
systematic effect due to cluster projection. Even though, cluster
projection had been suggested as a possible systematic bias in
measurements, more attention has recently been
given to various *exotic* effects as a way to explain the broad
distribution of Hubble constant values. We have shown here the
presence of projection effects in the present
and measurements and have analytically
calculated the effect of cluster projection in deriving the Hubble
constant. It is also assumed in literature that for a large sample of
clusters, the average of the individual Hubble constants, after making
various corrections, can be used to determine the true Hubble
constant. We have shown here that this may not be easily possible, and
that when a random and large sample is available with a mix of prolate
and oblate clusters, the best that one could expect to obtain is a
Hubble constant value within 10% of the true value, unless the
distribution of ellipticities for cluster sample is carefully taken
into account. Thus, we strongly recommend that more attention be given
to the cluster asphericity in deriving cosmologically important
measurements such as Hubble constant and the cluster gas mass
fraction. For individual clusters, for which SZ observations are
available, we have shown that a combined study of SZ, X-ray, velocity
dispersion measurements and weak lensing observations can be used in a
more physical manner to derive the Hubble constant. Thus, we have
demonstrated the usefulness of gravitational lensing observations of
galaxy clusters for cosmologically important studies, and when
combined, more meaningful results are expected to be produced instead
of just combining SZ and X-ray observations. We strongly recommend
that weak lensing observations and velocity dispersion measurements be
carried out to test the reliability of Hubble constant values in Table
1, and to complement SZ observations of clusters.
© European Southern Observatory (ESO) 1998
Online publication: October 21, 1998
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