.jpg)
Inastronomy, theintracluster medium (ICM) is the superheatedplasma that permeates agalaxy cluster. The gas consists mainly ofionized hydrogen andhelium and accounts for most of thebaryonic material in galaxy clusters. The ICM is heated to temperatures on the order of 10 to 100megakelvins, emitting strongX-ray radiation.
The ICM is composed primarily of ordinarybaryons, mainly ionised hydrogen and helium.[1] This plasma is enriched with heavier elements, includingiron. The average amount of heavier elements relative to hydrogen, known as themetallicity in astronomy, ranges from a third to a half of the value in theSun.[1][2] 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.[2] In some clusters (e.g. theCentaurus Cluster) the metallicity of the gas can rise to above that of the sun.[3] Due to thegravitational field of clusters, metal-enriched gas ejected fromsupernova remainsgravitationally bound to the cluster as part of the ICM.[2] By looking at varyingredshift, 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.[4]
Roughly 15% of a galaxy cluster's mass resides in the ICM. The stars and galaxies contribute only around 5% to the total mass. It is theorized that most of the mass in a galaxy cluster consists ofdark matter and not baryonic matter. For theVirgo Cluster, the ICM contains roughly 3 × 1014 M☉ while the total mass of the cluster is estimated to be 1.2 × 1015 M☉.[1][5]
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. Themean 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.[6]
As the ICM is at such high temperatures, it emitsX-ray radiation, mainly by thebremsstrahlung process and X-rayemission lines from the heavy elements.[1] These X-rays can be observed using anX-ray telescope and through analysis of this data, it is possible to determine the physical conditions, including the temperature, density, and metallicity of the plasma.
Measurements of the temperature and density profiles in galaxy clusters allow for a determination of the mass distribution profile of the ICM throughhydrostatic equilibrium modeling. The mass distributions determined from these methods reveal masses that far exceed the luminous mass seen and are thus a strong indication of dark matter in galaxy clusters.[7]
InverseCompton scattering of low energy photons through interactions with the relativistic electrons in the ICM cause distortions in the spectrum of thecosmic microwave background radiation (CMB), known as theSunyaev–Zel'dovich effect. These temperature distortions in the CMB can be used by telescopes such as theSouth Pole Telescope to detect dense clusters of galaxies at high redshifts.[8]
In December 2022, theJames Webb Space Telescope is reported to be studying the faint light emitted in the intracluster medium.[9] A 2018 study found this to be an "accurate luminous tracer of dark matter".[10]
Plasma in regions of the cluster, with a cooling time shorter than the age of the system, should be cooling due to strong X-ray radiation where emission is proportional to the density squared. Since the density of the ICM is highest towards the center of the cluster, theradiative cooling time drops a significant amount.[11] The central cooled gas can no longer support the weight of the external hot gas and thepressure gradient drives what is known as acooling flow where the hot gas from the external regions flows slowly towards the center of the cluster. This inflow would result in regions of cold gas and thus regions of newstar formation.[12] Recently however, with the launch of new X-ray telescopes such as theChandra X-ray Observatory, images of galaxy clusters with better spatial resolution have been taken. These new images do not indicate signs of new star formation on the order of what was historically predicted, motivating research into the mechanisms that would prevent the central ICM from cooling.[11]
There are two popular explanations of the mechanisms that prevent the central ICM from cooling: feedback fromactive galactic nuclei through injection ofrelativistic jets of plasma[13] and sloshing of the ICM plasma during mergers with subclusters.[14][15] The relativistic jets of material from active galactic nuclei can be seen in images taken by telescopes with high angular resolution such as theChandra X-ray Observatory.