Intracluster medium

In astronomy, the intracluster medium (ICM) is the superheated plasma that permeates a galaxy cluster. The gas consists mainly of ionized hydrogen and helium and accounts for most of the baryonic material in galaxy clusters. The ICM is heated to temperatures on the order of 10 to 100 megakelvins, emitting strong X-ray radiation. The ICM is composed primarily of ordinary baryons, mainly ionised hydrogen and helium. This plasma is enriched with heavier elements, including iron. The average amount of heavier elements relative to hydrogen, known as metallicity in astronomy, ranges from a third to a half of the value in the sun. Studying the chemical composition of the ICMs as a function of radius has shown that cores of the galaxy clusters are more metal rich than at larger radii. In some clusters (e.g. the Centaurus cluster) the metallicity of the gas can rise to above that of the sun. Due to the gravitational field of clusters, metal-enriched gas ejected from supernova remains gravitationally bound to the cluster as part of the ICM. By looking at varying redshift, which corresponds to looking at different epochs of the evolution of the Universe, the ICM can provide a history record of element production in a galaxy. Roughly 10% of a galaxy cluster's mass resides in the ICM. The stars and galaxies may contribute only 1% to the total mass. It is theorized that most of the mass in a galaxy cluster consists of dark matter and not baryonic matter. For the Virgo Cluster, the ICM contains roughly 3 × 1014 M☉ while the total mass of the cluster 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.