Galaxy clusters are the largest stable systems in the Universe. They can contain thousands of galaxies all held together by the glue of gravity. A spectacular new image from the NASA/ESA Hubble Space Telescope shows one of these colossal systems — a galaxy cluster called SDSS J0333-0651.
At first glance, this Hubble image is dominated by the glow of the swirling spiral to the lower left of the frame. However, this galaxy is far from the most interesting spectacle here — behind it sits the galaxy cluster SDSS J0333-0651. Image credit: NASA / ESA / Hubble.
Galaxies are not randomly distributed in space; they swarm together, gathered up by the unyielding hand of gravity, to form clusters.
At one point in time galaxy clusters were believed to be the largest structures in the Universe — until they were usurped in the 1980s by the discovery of superclusters.
However, clusters do have one thing to cling on to; superclusters are not held together by gravity, so galaxy clusters still retain the title of the biggest structures in the Universe bound by gravity.
Galaxy clusters are ‘laboratories’ for studying the relationship between the distributions of dark and visible matter.
In 1937, Swiss astronomer Fritz Zwicky realized that the visible component of a galaxy cluster represents only a tiny fraction of the total mass. About 80-85% of the matter is invisible, the so-called ‘dark matter.’
Galaxy clusters are gravitationally dominated by dark matter but also contain vast quantities of hot gas.
This gas cools by emitting X-ray radiation, decreasing its temperature and allowing more gas to flow to the center.
The galaxy at the center of a cluster — so-called brightest cluster galaxy (BCG) — sits at the center of the dark matter halo, where also the gas density is highest.
Furthermore, BCGs assembled via many mergers of mostly cluster galaxies.
SDSS J0333-0651 was imaged as part of a study of star formation in distant BCGs.
