Optical and X-ray structures of galaxy clusters. I

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 clusters. 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.