Abstract
In the Local Group, the best estimate of mass is given by the Viriel theorem. Compact groups pose the problem of their lifetime. In fact, according to simulations, they have not been compact for a long time. In galaxy clusters, there are several methods for determining mass : Viriel, but galaxy velocities take a long time to obtain ; hydrostatic equilibrium of hot gas, emitting X(if equilibrium) ; strong gravitational lenses ; gravitational shear. In the case of violent collisions between two clusters, dark matter may separate from gas, which contains the bulk of the baryons. We describe the scaling relationships between luminosity, temperature and cluster mass.
