Gravity creates a huge variety of cosmic structure, but most galaxies are found in groups of no more than a few dozen members. Groups range from the satellite systems of giant galaxies to loose associations a few times denser than their surroundings to compact cores of rich galaxy clusters. The common thread linking these examples is that the galaxies making up a group interact more with each other than they do with the rest of the universe; thus a group is a dynamical unit.
Groups are important for the evolution of galaxies and of large-scale structures. Galaxy formation appears to be a drawn-out process, involving the collapse of primordial perturbations, accretion of gas and dark matter, outright merging of distinct objects, and outflows of gas enriched by supernovae. Most galaxies conduct these transactions in group environments. But group environments are unstable; while the galaxies in a group are forming, the group itself may be separating out from the cosmic expansion, collapsing under the influence of gravity, accreting new members, and finally merging with other groups to build clusters and superclusters.
We can study the present outcome of these ongoing processes in our immediate vicinity and glimpse some earlier stages at high redshift. However, reconstructing the evolution of groups is a difficult problem. Two powerful tools which complement optical studies are computer simulations and multi-wavelength observations. Simulations model the key dynamical ingredients of groups, including the dark matter which is otherwise inaccessible. Observations at radio and X-ray wavelengths show how intergalactic gas responds as groups evolve.
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Last modified: April 18, 2001