Composition
The ICM is composed primarily of ordinary baryons (mainly ionised hydrogen and helium). This plasma is enriched with heavy elements, such as iron. The amount of heavy elements relative to hydrogen (known as metallicity in astronomy) is roughly a third of the value in the sun. Most of the baryons in the cluster (80-95%) reside in the ICM, rather than in the luminous matter, such as galaxies and stars. However, most of the mass in a galaxy cluster consists of dark matter.
Although the ICM on the whole contains the bulk of a cluster's baryons, it is not very dense, with typical values of 10−3 particles per cubic centimeter. The mean free path of the particles is roughly 1016 m, or about one lightyear.
The strong gravitational field of clusters means that they can retain even elements created in high-energy supernovae. Studying the composition of the ICM at varying redshift (which results in looking at different points back in time) can therefore give a record of element production in the universe if they are typical.
Read more about this topic: Intracluster Medium
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