Galaxy Groups

Galaxy groups are the smallest gravitationally bound collections of galaxies. They typically contain 3 to 50 galaxies within a region spanning a few million light-years. Most galaxies in the universe, including our own Milky Way, belong to galaxy groups rather than larger galaxy clusters or existing in isolation.

The Local Group is our cosmic neighborhood. It contains about 80 known galaxies dominated by three large spirals: the Milky Way, Andromeda (M31), and Triangulum (M33). The rest are dwarf galaxies - small, faint systems containing millions to billions of stars rather than hundreds of billions. The entire group spans roughly 10 million light-years with the Milky Way and Andromeda as the two main gravitational anchors separated by 2.5 million light-years.

Galaxy groups are dominated by dark matter. The visible galaxies account for only a small fraction of the total mass. Dark matter provides the gravitational glue holding the group together. The mass-to-light ratio in groups is typically 100 to 300 times that of the sun, meaning most of the mass is invisible. This dark matter forms an extended halo encompassing all the group members.

The dynamics within groups are gentler than in massive galaxy clusters but still dramatic over cosmic timescales. Galaxies within a group orbit their common center of mass at speeds of 100 to 300 kilometers per second. Close encounters are common. The Magellanic Clouds, two dwarf irregular galaxies orbiting the Milky Way, show tidal distortions from their interaction with our galaxy. The Magellanic Stream, a ribbon of hydrogen gas trailing behind them, was stripped away by tidal forces.

Galaxy groups evolve through mergers. When galaxies collide, they don’t actually crash star-to-star - the distances between stars are too vast. Instead, gravitational interactions scramble their structures and trigger intense bursts of star formation as gas clouds collide and compress. Eventually the galaxies merge into a single larger galaxy. The Andromeda Galaxy shows evidence of having cannibalized several smaller galaxies in the past billion years.

The Milky Way and Andromeda are approaching each other at 110 kilometers per second. In roughly 4.5 billion years they’ll collide and merge, probably absorbing Triangulum as well. Computer simulations show the collision will take a billion years or more, with multiple close passages before the final merger. The resulting galaxy will be a giant elliptical, nicknamed “Milkomeda” by some astronomers.

Compact groups show more dramatic interactions. Stephan’s Quintet contains five galaxies packed into a region smaller than the Milky Way’s diameter. Four of them are interacting violently, with tidal tails of stars and gas stretching between them. The collisions have triggered intense star formation and created shock waves heating intergalactic gas to millions of Kelvin. The fifth galaxy is actually a foreground object seven times closer, not part of the interacting system.

BTW the distribution of galaxies within groups isn’t random. Spiral galaxies tend to occupy the outer regions where tidal interactions are weaker. Elliptical galaxies and irregular galaxies resulting from past mergers cluster toward the group center. This segregation reflects the violent history of the group - galaxies in the center have undergone more interactions and mergers.

Galaxy groups are fossil records of cosmic structure formation. They formed when slight density fluctuations in the early universe grew under gravity, pulling matter together. The Local Group likely assembled over billions of years as smaller groups merged and individual galaxies fell in. Studying the kinematics and distribution of group members reveals this assembly history.

The intergalactic medium within groups contains hot, diffuse gas at temperatures around one million Kelvin. This gas emits faint X-rays detectable by space telescopes. The gas mass is comparable to the total mass in stars within all the group’s galaxies. Some of this gas is primordial material that never condensed into galaxies. Some was expelled from galaxies by supernova-driven winds. This enriched gas will eventually fall back onto galaxies, fueling future generations of star formation.