Galaxy Cluster
Galaxy clusters are the most massive gravitationally bound structures in the Universe, containing hundreds to thousands of galaxies, vast amounts of hot gas, and a dominant component of dark matter. Typical masses range from 10¹⁴ to 10¹⁵ M☉ (solar masses), and sizes span several million light‑years.
These clusters form through hierarchical growth: smaller galaxy groups merge over time to create larger clusters. They are the primary building blocks of the cosmic web and serve as anchors for superclusters.
Most of a cluster’s mass resides in dark matter, inferred from galaxy motions and gravitational lensing. The intracluster medium (ICM) is made of hot plasma at tens of millions of Kelvin, emitting X‑rays observable by telescopes like Chandra and XMM‑Newton.
Galaxy clusters reveal fundamental physics. Fritz Zwicky first inferred dark matter in the 1930s by noting galaxies moved too quickly to be held by visible mass. Detailed X‑ray and lensing studies confirm that visible galaxies make up only ~1%, while ICM is ~9% and dark matter comprises ~90% of cluster mass.
Clusters are cosmological tools. Measurements of their number, mass distribution, and growth trace the nature of dark energy and the evolution of large-scale structure.
They also act as gravitational lenses: their mass bends light from distant galaxies, magnifying and distorting background sources—used to study high-redshift galaxies and map mass distribution.
Galaxy clusters include familiar nearby examples like the Virgo Cluster, Coma Cluster, and the Perseus Cluster. They also host extreme systems such as the Bullet Cluster—where colliding subclusters provide compelling evidence for dark matter—and distant massive clusters located over 10 billion light‑years away.
Modern observatories like Hubble, Chandra, and JWST continue to probe cluster dynamics, star formation, black hole growth, and dark matter profiles within these colossal systems.
Sources
APODs including "Galaxy Cluster"
Giant Cluster Bends, Breaks Images
23 August 2015
What are those strange blue objects? Many of the brightest blue images are of a single, unusual, beaded, blue, ring-like galaxy which just happens to line-up behind a giant cluster of galaxies. Cluster galaxies here typically appear yellow and -- together with the cluster's dark matter -- act as a gravitational lens. A gravitational lens can create several images of background galaxies, analogous to the many points of light one would see while looking through a wine glass at a distant street light. The distinctive shape of this background galaxy -- which is probably just forming -- has allowed astronomers to deduce that it has separate images at 4, 10, 11, and 12 o'clock, from the center of the cluster. A blue smudge near the cluster center is likely another image of the same background galaxy. In all, a recent analysis postulated that at least 33 images of 11 separate background galaxies are discernable. This spectacular photo of galaxy cluster CL0024+1654 from the Hubble Space Telescope was taken in November 2004.