Galaxies are not isolated. They form groups and clusters through gravity and, on the largest cosmic scales, create a bubble-like large-scale structure.
Basic Knowledge
The hierarchical structure of galaxies:
・Galaxy groups (several to several dozen galaxies; the Local Group is an example)
・Galaxy clusters (several dozen to several thousand galaxies; several million to several tens of millions of light-years in diameter; the Virgo Cluster is an example)
・Superclusters (huge structures containing several dozen galaxy clusters; the Laniakea Supercluster, about 500 million light-years in diameter)
In the large-scale structure of the universe, galaxies are distributed in filaments (threadlike structures) and sheets, with voids (empty regions containing almost no galaxies) between them. This is called the cosmic bubble structure (cosmic web).
Most of the mass of a galaxy cluster (70〜90%) consists of dark matter, and the hot intracluster medium (ICM) between galaxies emits X-rays.
Galaxy cluster (a group of several dozen to several thousand galaxies gravitationally bound together; dominated by dark matter)
Supercluster (a collection of galaxy clusters; several hundred million light-years in diameter; the Laniakea Supercluster is a local example)
Filament (the threadlike framework of large-scale structure, along which galaxies and galaxy groups are connected)
Void (an empty region containing almost no galaxies; some are several hundred million light-years in diameter)
Cosmic bubble structure (the cosmic web consisting of filaments + sheets + voids)
Intracluster medium (ICM) (extremely hot plasma between galaxies; shines in X-rays; about 15% of the mass of a galaxy cluster)
Further Study (Background and Significance)
Evidence for dark matter in galaxy clusters dates back to observations in the 1930s by Zwicky of the speeds of galaxies moving within clusters. Galaxies were moving much faster than expected from the apparent mass of the clusters (estimated from luminosity), suggesting that a large amount of invisible mass was present (an application of the virial theorem).
Observations of gravitational lensing (the phenomenon in which massive objects bend background light) are also a powerful way to map the distribution of dark matter in galaxy clusters. In the Bullet Cluster (the remnant of a collision between two galaxy clusters), dark matter is concentrated in positions different from the X-ray-emitting ICM (ordinary matter), providing decisive evidence that the two exist independently.
The large-scale structure of the universe formed when quantum fluctuations just after the Big Bang were stretched by cosmic expansion and grew through gravity. The seeds of the initial fluctuations can be directly observed from patterns of temperature fluctuations in the CMB.
- Hierarchy: galaxy group＜galaxy cluster＜supercluster
- Laniakea Supercluster: includes the Local Group; about 500 million light-years in diameter
- Cosmic bubble structure: filaments + sheets + voids
- Most of a galaxy cluster (70〜90%) is dark matter
- Intracluster medium (ICM) emits X-rays
- Gravitational lensing maps the distribution of dark matter
- The Bullet Cluster is decisive evidence for the independent existence of dark matter
Points of Caution (Easily Confused)
① Galaxy groups (fewer than several dozen galaxies) and galaxy clusters (several dozen to several thousand galaxies) differ in scale. ② A void is not completely empty; it is a region with extremely few galaxies. ③ Gravitational lensing concerns the bending of light and is used to detect dark matter; it is different from a lensing galaxy. ④ The ICM (intracluster medium) shines in X-rays, not visible light.
Practice
- What is the hot plasma between galaxies in a galaxy cluster called?
- What is the empty region containing almost no galaxies in the large-scale structure of the universe called?
- The Bullet Cluster observation is considered evidence for the existence of what?