The intracluster medium is diffuse hot plasma between the galaxies of a galaxy cluster. Its X-ray radiation includes thermal bremsstrahlung and atomic line emission. Measured temperature and density profiles supply a cluster hydrostatic mass estimator when approximately thermal hydrostatic equilibrium holds. The medium supplies much of a cluster's baryonic mass, but this need not equal its total gravitating mass.
For spherical thermal hydrostatic equilibrium, . Inserting the ideal gas law with constant mean molecular weight gives the displayed estimator. The density is gas density, while includes every gravitating component. A varying adds inside the logarithmic bracket. Nonthermal pressure requires differentiating the total supporting pressure rather than thermal pressure alone.
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The Intracluster Medium (ICM) refers to the hot, diffuse gas that exists between the galaxies in a galaxy cluster. This medium is primarily composed of ionized hydrogen and helium, along with trace amounts of heavier elements. The ICM has an average temperature ranging from a few million to tens of millions of degrees Celsius and is generally at a very low density, but it contains a significant amount of the total mass of the cluster.