Galaxy Groups & Clusters: Observation, Measurement & Evidence

A cluster includes member galaxies, diffuse intracluster plasma, dark matter, and often a massive central galaxy. Groups are less massive and have lower velocity dispersions, making galaxy mergers relatively easier.

Galaxy redshifts and velocities reveal bound populations, X-ray emission maps hot gas, gravitational lensing maps total mass, and the motions of galaxies require far more mass than visible stars alone provide.

Clusters grow through gravitational collapse and hierarchical merging of smaller halos. Galaxies orbit in the shared potential, while gas falling into the deep well is shock-heated to X-ray temperatures. Environmental processes strip, heat, or disturb gas in member galaxies.

Redshifts identify bound members, X-rays map hot gas, lensing maps total mass, and velocity dispersion estimates dynamics. Agreement among these probes anchors cluster mass measurements.