Galaxy group

Characteristics

Galaxy groups occupy an intermediate place between isolated galaxies and rich galaxy clusters. Their observational and physical properties include:

• Membership: usually a handful up to a few dozen galaxies, with the precise cutoff between a group and a cluster varying in different studies.
• Size and scale: typical diameters are around one to a few megaparsecs, depending on how membership is defined.
• Mass and dark matter: total mass is dominated by dark matter; combined baryonic and dark matter masses are often many billions to trillions of solar masses.
• Dynamics: member galaxies orbit a common center and interact through mutual gravity, leading to mergers, tidal stripping and the exchange of gas.
• Intragroup medium: some groups contain a diffuse, hot gas detectable in X-rays, though typically less dense and cooler than the intracluster medium of rich clusters.

Formation and evolution

Groups form through the hierarchical assembly of matter in the cosmic web: smaller dark matter halos merge and accrete into larger systems. Within groups, interactions are frequent, which can transform galaxy shapes and star formation rates. Gas may be removed from disks by tidal forces or funneled into central regions, and small satellites can be accreted by larger galaxies, building up their mass over time.

Some groups evolve into more relaxed systems called fossil groups, where a single large galaxy dominates the luminosity because it has merged with many former companions. The boundary between a large group and a small cluster is not sharp; collections of galaxies larger than groups are generally referred to as galaxy clusters or superclusters, which are discussed further at clusters and superclusters.

Observational identification

Astronomers identify groups using redshift surveys and positional data, often applying algorithms such as friends‑of‑friends to group galaxies with similar velocities and positions. X-ray observations can reveal hot intragroup gas, and gravitational lensing or satellite dynamics provide estimates of total mass. Catalogs and surveys compile group properties for statistical studies; see representative survey resources at survey resources.

Examples and significance

The Local Group is the best-known example: it contains the Milky Way (Milky Way), the Andromeda galaxy, the Triangulum galaxy, and more than forty smaller dwarf galaxies (Local Group). Other nearby groups, such as the M81 Group and the Sculptor Group, provide accessible laboratories for studying galaxy interactions and the role of environment in galaxy evolution.

Because most galaxies in the nearby universe reside in groups rather than in rich clusters, groups are the dominant environment for the study of galaxy growth, dark matter distribution, and baryon cycling. Their relatively low velocity dispersions make mergers more common than in clusters, so groups are key sites for the morphological transformation of galaxies and for understanding the hierarchical build-up of structure in the cosmos.