astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...

, the intracluster medium (ICM) is the superheated plasma that permeates a

galaxy cluster A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. They are the second-lar ...

. The gas consists mainly of

ionized hydrogen A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particle ...

and

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...

and accounts for most of the

baryon In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classifie ...

ic material in galaxy clusters. The ICM is heated to temperatures on the order of 10 to 100 megakelvins, emitting strong

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

radiation.

Composition

The ICM is composed primarily of ordinary

s, mainly ionised hydrogen and helium. This plasma is enriched with heavier elements, including

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

. The average amount of heavier elements relative to hydrogen, known as

metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as ...

in astronomy, ranges from a third to a half of the value in the

sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

. 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 A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or whe ...

remains

gravitationally bound The gravitational binding energy of a system is the minimum energy which must be added to it in order for the system to cease being in a gravitationally bound state. A gravitationally bound system has a lower (''i.e.'', more negative) gravitati ...

to the cluster as part of the ICM. By looking at varying

redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in fr ...

, 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 Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ...

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 In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as ...

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.

Observing the intracluster medium

As the ICM is at such high temperatures, it emits

radiation, mainly by the

bremsstrahlung ''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typical ...

process and X-ray

emission lines A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to ident ...

from the heavy elements. These X-rays can be observed using an

X-ray telescope An X-ray telescope (XRT) is a telescope that is designed to observe remote objects in the X-ray spectrum. In order to get above the Earth's atmosphere, which is opaque to X-rays, X-ray telescopes must be mounted on high altitude rockets, balloon ...

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 through

hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planeta ...

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. Inverse

Compton scattering Compton scattering, discovered by Arthur Holly Compton, is the scattering of a high frequency photon after an interaction with a charged particle, usually an electron. If it results in a decrease in energy (increase in wavelength) of the photon ...

of low energy photons through interactions with the relativistic electrons in the ICM cause distortions in the spectrum of the cosmic microwave background radiation (CMB), known as the Sunyaev–Zel'dovich effect. These temperature distortions in the CMB can be used by telescopes such as the

South Pole Telescope The South Pole Telescope (SPT) is a diameter telescope located at the Amundsen–Scott South Pole Station, Antarctica. The telescope is designed for observations in the microwave, millimeter-wave, and submillimeter-wave regions of the electroma ...

to detect dense clusters of galaxies at high redshifts. In December 2022, the

James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope which conducts infrared astronomy. As the largest optical telescope in space, its high resolution and sensitivity allow it to view objects too old, distant, or faint for the Hubble Sp ...

is reported to be studying the faint light emitted in the intracluster medium.

Cooling flows

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, the radiative cooling time drops a significant amount. The central cooled gas can no longer support the weight of the external hot gas and the pressure gradient drives what is known as a cooling 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 new star formation. Recently however, with the launch of new X-ray telescopes such as the

Chandra X-ray Observatory The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources ...

, 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.

Heating

NASA-PerseusGalaxyCluster-ChandraXRayObservatory-20140624

There are two popular explanations of the mechanisms that prevent the central ICM from cooling: feedback from

active galactic nuclei An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not prod ...

through injection of

relativistic jets An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as an extended beam along the axis of rotation. When this greatly accelerated matter in the beam approaches the speed of light, astrophysical jets bec ...

of plasma and sloshing of the ICM plasma during mergers with subclusters. The relativistic jets of material from active galactic nuclei can be seen in images taken by telescopes with high angular resolution such as the

References

{{DEFAULTSORT:Intracluster Medium * Large-scale structure of the cosmos Extragalactic astronomy Medium, intracluster Space plasmas Plasma physics Intergalactic media

Composition

Observing the intracluster medium

Cooling flows

Heating

See also

References