Intracluster media

In astronomy, the intracluster medium (ICM) is the superheated plasma that permeates a galaxy cluster. This gas is heated to temperatures on 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 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. The metallicity rises from the outer region towards the center and in some clusters (e.g. the Centaurus cluster) the metallicity of the gas can rise above that of the sun. Due to the strong gravitational field of clusters, metal-enriched gas ejected during supernovae can remain gravitationally bound to the cluster and become part of the ICM. 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.

Most of the baryons in the cluster (80-95%) reside in the ICM (for the Virgo cluster, hot gasses make up roughly 3 × 10¹⁴ M_☉), rather than in the luminous matter, such as galaxies and stars. However, most of the mass in a galaxy cluster consists of dark matter (for the Virgo Cluster the total mass is estimated to be 1.2 × 10¹⁵ M_☉).

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 density of the ICM rises towards the centre of the cluster with a relatively strong peak. In addition, the temperature of the ICM typically drops to 1/2 or 1/3 of the outer value in the central regions. Once the density of the plasma reaches a critical value, enough interactions between the ions ensures cooling via X-ray radiation.

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