Intracluster medium

In astronomy, the intracluster medium (ICM) is the superheated plasma present at the center of a galaxy cluster. This gas is heated to temperatures of the order of 10 to 100 megakelvins and composed mainly of ionized hydrogen and helium, containing most of the baryonic material in the cluster. The ICM strongly emits X-ray radiation.

The ICM is heated to high temperatures by the gravitational energy released by the formation of the cluster from smaller structures.Kinetic energy gained from the gravitational field is converted to thermal energy by shocks. The high temperature ensures that the elements present in the ICM are ionised. Light elements in the ICM have all the electrons removed from their nuclei.

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.

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