Intracluster medium
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In astronomy, the intracluster medium (or ICM) is the superheated gas present at the center of a galaxy cluster.
The ICM is composed primarily of baryons, i.e. ordinary matter; most of a cluster's baryons (about 80-95%) reside in the ICM, rather than in the luminous matter, such as galaxies and stars (but note that most of the cluster's overall mass is still dark matter -- the ICM is merely the bulk of the baryonic matter). Because clusters contain so much mass, in both baryons and dark matter, clusters have extremely strong gravitational fields, so that matter which falls into the center of a cluster from farther out will gain significant gravitational potential energy as it falls in. This is thought to be the primary mechanism responsible for the high temperature of the ICM gas, which can be as high as 107 or 108 kelvins.
Although the ICM on the whole contains the bulk of a cluster's baryons, it is not very dense, with typical values of 0.01 electron per cubic centimeter. This low density makes the term "gas" somewhat misleading for describing the phase of the particles; they are so spread out that they do not really interact. The intense heat ensures that elements present in the ICM are ionized; for lighter elements, this means that they may have no electrons (i.e. they are totally ionized), while for heavier elements, many of the outer electrons will be stripped.
Clusters' strong gravity means they can retain even elements created in high-energy supernovae. Studying the ICM's composition at varying redshift (which results in looking at different points back in time) can therefore give a record of element production in the universe.
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