The differences between optical and X-ray structures of galaxy clusters are discussed. We analyse in detail 7 Abell clusters. There is an increasing number of arguments in favour of different distributions of galaxies, gas, and dark matter in many dusters. We argue that most clusters present sub-structures at least in the gaseous and galactic components and, moreover, the subclustering of different components does not always coincide. Such arguments strongly support the idea that most galaxy clusters are by far more complex systems than accepted until now and are usually not in an hydrostatic isothermal equilibrium.Key words: galaxy clusters -structures -dominant galaxies A A A subject classification: 160
IntroductionGalaxy clusters are normally characterized by the BM types I, 11, and I11 or by the RS types cD, B, L, F, and I. Correlations between these morphological cluster types and the physical properties of the clusters are expected. This expectation concerns among others the X-ray properties. However many authors find only marginal evidence for such a correlation. In this connection the question arises which kind of observation is better suited for the description of the physical properties of galaxy clusters. Very often it is argued that optical observations are plagued by projection effects and by small number statistics. It is assumed that X-ray observations of clusters can overcome these problems because the relaxation time-scale of the X-ray emitting gas is much shorter than the age of a cluster. Thus, it is usually assumed that the gas is relaxed and the X-ray emission can be used as a tracer of the gravitational potential of clusters. A second point is that many galaxy clusters are observed to be commonly bright in X-rays due to the high gas densities in their cores. Therefore, they should be detected out to large redshifts making them useful probes of cosmological studies.Since all components of a given cluster share the total gravitational potential, it is assumed that both the intra-cluster medium and galaxy number densities are related to the potential well of the cluster in a similar way, or that dark matter, hot gas, and galaxy distribution evolve in a tightly coupled way (Frenk et al. 1995). Traditionally an hydrostatic equilibrium of both the hot intra-cluster gas and the galaxies with the binding cluster potential is assumed (e.g., Wu 1994, Lubin and Bahcalll993).In connection with all these theoretical considerations the distributions of different components in clusters (galaxies, gas, and dark matter) should show the same behaviour. The usefulness of the observation of one or more components is considered to be determined only by the possibility of observations. In principle, results from all such kinds of observations -galaxy distribution or X-ray emissivity, for example -should lead to equivalent results.However, the very observation of sub-structures in the majority of rich clusters, the different scale lengths of the various components in a cluster, or the so-called &problem, point to ...