The Dynamics of Groups and Clusters of Galaxies and Links to Cosmology
The links between the internal structure of galaxy groups and clusters and cosmological parameters are reviewed here. The mass density profiles of clusters, inferred from both optical analyses of the galaxy surface number density profile coupled with internal kinematics, and from weak gravitational lensing, appear cuspy with inner density profiles at least as steep as $r^{-1}$, as is expected from high-resolution cosmological $N$-body simulations starting with scale-free or CDM initial conditions. The high level of substructure both in the galaxy distribution and in the diffuse hot gas seen in X-rays is consistent with dynamically young clusters and a cosmological density parameter $Ω_0 \gta 0.5$. Despite the importance of cluster-cluster merging witnessed by these substructures, the spherical top-hat cosmology appears to give good indications of the underlying physics of clusters and groups, including a fundamental evolutionary track in a space of observational parameters for groups. Dynamical processes operating in groups and clusters are reviewed. Previrialization of groups and clusters before their full collapse is not expected on theoretical grounds, and the ensemble of positions of compact groups, relative to the fundamental track is a potentially useful constraint on previrialization.