Clusters of galaxies are the largest stable systems in the Universe.
They are like 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 — the thousands of millions of stars in each of the thousands of galaxies — 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 sits at the center of the dark matter halo, where also the gas density is highest.
Furthermore, these so-called ‘brightest cluster galaxies’ assembled via many mergers of mostly cluster galaxies.
The color image of the massive galaxy cluster PSZ2 G138.61-10.84 was taken by Hubble’s Advanced Camera for Surveys (ACS) and Wide-Field Camera 3 (WFC3) as part of an observing program called Reionization Lensing Cluster Survey(RELICS).
RELICS imaged 41 giant galaxy clusters over the course of 390 Hubble orbits, aiming to find the brightest distant galaxies.
Studying these galaxies in more detail with both current telescopes and the forthcoming NASA/ESA/CSA James Webb Space Telescope will hopefully tell us more about our cosmic origins.