This NASA/ESA Hubble Space Telescope image shows a massive galaxy cluster glowing brightly in the darkness. Despite its beauty, this cluster bears the distinctly unpoetic name of PLCK_G308.3-20.2.
Galaxy clusters can contain thousands of galaxies all
held together by the glue of gravity. At one point in time they were
believed to be the largest structures in the Universe — until they were
usurped in the 1980s by the discovery of superclusters,
which typically contain dozens of galaxy clusters and groups and span
hundreds of millions of light-years. 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.
One of the most interesting features of galaxy
clusters is the stuff that permeates the space between the constituent
galaxies: the intracluster medium
(ICM). High temperatures are created in these spaces by smaller
structures forming within the cluster. This results in the ICM being
made up of plasma — ordinary matter in a superheated state. Most
luminous matter in the cluster resides in the ICM, which is very
luminous X-rays. However, the majority of the mass in a galaxy cluster exists in the form of non-luminous dark matter.
Unlike plasma, dark matter is not made from ordinary matter such as
protons, neutrons and electrons. It is a hypothesised substance thought
to make up 80 % of the Universe’s mass, yet it has never been directly
observed.
This image was taken by Hubble’s Advanced Camera for Surveys and Wide-Field Camera 3 as part of an observing programme called RELICS (Reionization Lensing Cluster Survey). RELICS imaged 41 massive galaxy clusters with the aim of finding the brightest distant galaxies for the forthcoming NASA/ESA/CSA James Webb Space Telescope (JWST) to study.
Source: ESA /Hubble/Potw
