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Astron. Astrophys. 339, 623-628 (1998)

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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 [FORMULA] 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 [FORMULA] and [FORMULA] 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.

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© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998