Galaxy groups, clusters, and the cosmic web
Galaxies are not scattered randomly. They gather in groups and clusters, which form a vast cosmic web of filaments and voids.
The Local Group
The Milky Way belongs to the Local Group, about 80 or more galaxies within roughly 10 million light-years, dominated by the Milky Way and Andromeda, with the smaller Triangulum Galaxy (M33) and many dwarf galaxies.
Galaxy clusters
Galaxy clusters contain hundreds to thousands of galaxies bound by gravity. The Virgo Cluster, about 54 million light-years away, is the nearest large cluster; the Coma Cluster is a rich cluster about 320 million light-years away. Clusters are the largest gravitationally bound structures in the universe.
Hot gas in clusters
Most of a cluster's ordinary matter is not in galaxies but in hot gas between them, at tens of millions of kelvin, which glows in X-rays. Measuring this gas and galaxy motions shows clusters contain far more mass than visible matter, roughly five to ten times as much, another sign of dark matter.
Gravitational lensing
Massive clusters bend light from galaxies behind them, producing arcs and multiple images, as predicted by general relativity. Lensing lets astronomers map the total mass of clusters, including dark matter, and magnifies very distant galaxies, acting as natural telescopes.
The Bullet Cluster
In the Bullet Cluster, two clusters collided. The hot gas, which makes up most ordinary matter, was slowed and left in the middle, while lensing shows most mass passed through with the galaxies. This separation of mass from ordinary matter is considered strong evidence that dark matter is a real substance.
Superclusters and the cosmic web
Clusters and groups form superclusters, such as the Laniakea Supercluster, which contains the Local Group and over 100,000 galaxies across about 500 million light-years. On the largest scales, galaxies form a cosmic web of filaments and walls surrounding vast, nearly empty voids.
Galaxy interactions
Galaxies often collide and merge, especially in clusters. Mergers can trigger bursts of star formation, distort shapes into tidal tails, and turn spirals into ellipticals. The Antennae Galaxies are a famous example of colliding spirals.
Worked example
In a galaxy cluster, galaxies move at around 1,000 km/s. Using these speeds and the cluster's size in Newton's laws gives the mass needed to hold the cluster together. The result is far larger than the mass of all visible galaxies and gas, as Fritz Zwicky first found for the Coma Cluster in 1933, coining the term 'dunkle Materie' (dark matter).
Key takeaways
The Milky Way is in the Local Group, with Andromeda and Triangulum. Clusters such as Virgo and Coma contain hundreds to thousands of galaxies. Most of a cluster's ordinary matter is hot, X-ray-emitting gas, and clusters contain much more dark matter. Gravitational lensing maps mass. The Bullet Cluster separates dark matter from ordinary matter. Superclusters like Laniakea form a cosmic web of filaments and voids. Galaxy mergers transform shapes and trigger star formation.
Flashcard
About 80 or more nearby galaxies
Check your understanding
Why is the Bullet Cluster considered strong evidence for dark matter?