Galaxy groups and clusters

Galaxy groups and clusters are gravitationally bound systems of galaxies, ranging from small associations to massive clusters containing thousands of galaxies. Their galaxies, hot gas, and dark matter are held together chiefly by gravity.

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Dark matter halo: A gravitationally bound concentration of dark matter that provides the large-scale potential well for galaxies and gas. Most of a cluster’s mass lies in its extended dark matter halo.
Hierarchical structure formation: The cosmological process in which small gravitationally bound structures merge and grow into larger ones. Groups and clusters arise as smaller structures assemble into larger halos.
Sunyaev–Zeldovich effect: A change in cosmic microwave background photons caused by inverse Compton scattering off energetic electrons in hot gas. Its signal reveals hot cluster gas even when a cluster is distant.
Galaxy group: A relatively small gravitationally bound association of galaxies, usually less massive and less populated than a cluster. Groups occupy the lower-mass end of the systems covered here.
Intracluster medium: The hot, diffuse plasma filling the space between galaxies in a galaxy cluster. This plasma is a major component of clusters and emits X-rays.
Virialization: The process by which a gravitationally bound system settles toward a state described by the virial theorem. It links a system’s collapse to its characteristic motions and temperatures.
X-ray astronomy: The study of astronomical sources through X-rays emitted or absorbed in space. X-ray observations map the temperature and density of intracluster gas.
Galaxy cluster: A massive gravitationally bound system containing hundreds or thousands of galaxies and a large reservoir of hot gas. Clusters are the richest and most massive systems in this family.
Galaxy: A gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. Galaxies are the visible members whose motions trace the system’s gravity.
Accretion shock: A shock that heats infalling gas as it crosses into a gravitationally bound structure. Infalling gas can be heated to millions of degrees while joining a cluster.