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. [1] 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. [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. the Centaurus cluster) the metallicity of the gas can rise to above that of the sun. [3] Due to the gravitational field of clusters, metal-enriched gas ejected from supernova remains gravitationally bound to the cluster as part of the ICM. [2] 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. [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 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☉. [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. 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. [6]
As the ICM is at such high temperatures, it emits X-ray radiation, mainly by the bremsstrahlung process and X-ray emission lines from the heavy elements. [1] These X-rays can be observed using an X-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 through hydrostatic 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]
Inverse Compton scattering 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 to detect dense clusters of galaxies at high redshifts. [8]
In December 2022, the James Webb Space Telescope is reported to be studying the faint light emitted in the intracluster medium. [9] Which a 2018 study found 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, the radiative cooling time drops a significant amount. [11] 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. [12] Recently however, with the launch of new X-ray telescopes such as the Chandra 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 from active galactic nuclei through injection of relativistic 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 the Chandra X-ray Observatory.
The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. Galaxy formation is hypothesized to occur from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang. The simplest model in general agreement with observed phenomena is the Lambda-CDM model—that is, that clustering and merging allows galaxies to accumulate mass, determining both their shape and structure. Hydrodynamics simulation, which simulates both baryons and dark matter, is widely used to study galaxy formation and evolution.
Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation. They form the densest part of the large-scale structure of the Universe. In models for the gravitational formation of structure with cold dark matter, the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain ten to thousands of individual galaxies. The clusters themselves are often associated with larger, non-gravitationally bound, groups called superclusters.
Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars. As a branch of astronomy, star formation includes the study of the interstellar medium (ISM) and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products. It is closely related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations.
The Virgo Cluster is a large cluster of galaxies whose center is 53.8 ± 0.3 Mly away in the constellation Virgo. Comprising approximately 1,300 member galaxies, the cluster forms the heart of the larger Virgo Supercluster, of which the Local Group is a member. The Local Group actually experiences the mass of the Virgo Supercluster as the Virgocentric flow. It is estimated that the Virgo Cluster's mass is 1.2×1015M☉ out to 8 degrees of the cluster's center or a radius of about 2.2 Mpc.
Messier 87 is a supergiant elliptical galaxy in the constellation Virgo that contains several trillion stars. One of the largest and most massive galaxies in the local universe, it has a large population of globular clusters—about 15,000 compared with the 150–200 orbiting the Milky Way—and a jet of energetic plasma that originates at the core and extends at least 1,500 parsecs, traveling at a relativistic speed. It is one of the brightest radio sources in the sky and a popular target for both amateur and professional astronomers.
The Sunyaev–Zeldovich effect is the spectral distortion of the cosmic microwave background (CMB) through inverse Compton scattering by high-energy electrons in galaxy clusters, in which the low-energy CMB photons receive an average energy boost during collision with the high-energy cluster electrons. Observed distortions of the cosmic microwave background spectrum are used to detect the disturbance of density in the universe. Using the Sunyaev–Zeldovich effect, dense clusters of galaxies have been observed.
NGC 1275 is a type 1.5 Seyfert galaxy located around 237 million light-years away in the direction of the constellation Perseus. NGC 1275 is a member of the large Perseus Cluster of galaxies.
Radio halos are large-scale sources of diffuse radio emission found in the center of some, but not all, galaxy clusters. There are two classes of radio halos: mini-halos and giant radio halos. The linear size of giant radio halos is about 700kpc-1Mpc, whereas mini-halos are typically less than 500kpc. Giant radio halos are more often observed in highly X-ray luminous cluster samples than less luminous X-ray clusters in complete samples. They have a very low surface brightness and do not have obvious galaxy counterparts. However, their morphologies typically follow the distribution of gas in the intra-cluster medium. Mini-halos however, while similar to giant halos, are found at the center of cooling core clusters but around a radio galaxy.
The Perseus cluster is a cluster of galaxies in the constellation Perseus. It has a recession speed of 5,366 km/s and a diameter of 863′. It is one of the most massive objects in the known universe, containing thousands of galaxies immersed in a vast cloud of multimillion-degree gas.
The Bullet Cluster consists of two colliding clusters of galaxies. Strictly speaking, the name Bullet Cluster refers to the smaller subcluster, moving away from the larger one. It is at a comoving radial distance of 1.141 Gpc.
A cooling flow occurs when the intracluster medium (ICM) in the centres of galaxy clusters should be rapidly cooling at the rate of tens to thousands of solar masses per year. This should happen as the ICM is quickly losing its energy by the emission of X-rays. The X-ray brightness of the ICM is proportional to the square of its density, which rises steeply towards the centres of many clusters. Also the temperature falls to typically a third or a half of the temperature in the outskirts of the cluster. The typical [predicted] timescale for the ICM to cool is relatively short, less than a billion years. As material in the centre of the cluster cools out, the pressure of the overlying ICM should cause more material to flow inwards.
An intergalactic star, also known as an intracluster star or a rogue star, is a star not gravitationally bound to any galaxy. Although a source of much discussion in the scientific community during the late 1990s, intergalactic stars are now generally thought to have originated in galaxies, like other stars, before being expelled as the result of either galaxies colliding or of a multiple-star system traveling too close to a supermassive black hole, which are found at the center of many galaxies.
The Phoenix Cluster is a massive, Abell class type I galaxy cluster located at its namesake, southern constellation of Phoenix. It was initially detected in 2010 during a 2,500 square degree survey of the southern sky using the Sunyaev–Zeldovich effect by the South Pole Telescope collaboration. It is one of the most massive galaxy clusters known, with the mass on the order of 2×1015M☉, and is the most luminous X-ray cluster discovered, producing more X-rays than any other known massive cluster. It is located at a comoving distance of 8.61 billion light-years from Earth. About 42 member galaxies were identified and currently listed in the SIMBAD Astronomical Database, though the real number may be as high as 1,000.
RX J1347.5–1145 is one of the most massive galaxy clusters known discovered in X-rays with ROSAT. As a result, it is also one of the most X-ray-luminous because of its hot gas content. The object resides roughly 5 billion light-years away from the Solar System in the constellation of Virgo. Redshift was noted as z=0.451 with an X-ray luminosity of 1045 ergs s−1 in a paper from 2002. In 2013, one study found eight cases of the same object resulting from the intense gravitational bending of light, which makes it possible to identify a series of remote galaxies located inside the galaxy cluster with calculations from the photometric method between 5.5 and 7.5. That study made use of data from Cluster Lensing and Supernova survey with Hubble (CLASH) as well as other sources. The colors in the galaxy cluster are known to correspond with the level of brightness, or the number of electrons trapped in the examined wavelength range of the cluster, with the colors red, orange, and yellow as high intensity, blue-green and green as medium intensity, and blue and violet as low intensity. It is considered one of the brightest objects that is known by X-ray.
NGC 3862 is an elliptical galaxy located 300 million light-years away in the constellation Leo. Discovered by astronomer William Herschel on April 27, 1785, NGC 3862 is an outlying member of the Leo Cluster.
In cosmology, the missing baryon problem is an observed discrepancy between the amount of baryonic matter detected from shortly after the Big Bang and from more recent epochs. Observations of the cosmic microwave background and Big Bang nucleosynthesis studies have set constraints on the abundance of baryons in the early universe, finding that baryonic matter accounts for approximately 4.8% of the energy contents of the Universe. At the same time, a census of baryons in the recent observable universe has found that observed baryonic matter accounts for less than half of that amount. This discrepancy is commonly known as the missing baryon problem. The missing baryon problem is different from the dark matter problem, which is non-baryonic in nature.
NGC 4522 is an edge-on spiral galaxy located about 60 million light-years away within the Virgo Cluster in the constellation Virgo. NGC 4522 is losing its molecular gas though ram-pressure stripping as it plows though the cluster at a speed of more than 10 million kilometres per hour. The galaxy was discovered by astronomer John Herschel on January 18, 1828.
NGC 708 is an elliptical galaxy located 240 million light-years away in the constellation Andromeda and was discovered by astronomer William Herschel on September 21, 1786. It is classified as a cD galaxy and is the brightest member of Abell 262. NGC 708 is a weak FR I radio galaxy and is also classified as a type 2 Seyfert galaxy.
NGC 4636 is an elliptical galaxy located in the constellation Virgo. It is a member of the NGC 4753 Group of galaxies, which is a member of the Virgo II Groups, a series of galaxies and galaxy clusters strung out from the southern edge of the Virgo Supercluster. It is located at a distance of about 55 million light years from Earth, which, given its apparent dimensions, means that NGC 4636 is about 105,000 light years across.
NGC 5846 is an elliptical galaxy located in the constellation Virgo. It is located at a distance of circa 90 million light years from Earth, which, given its apparent dimensions, means that NGC 5846 is about 110,000 light years across. It was discovered by William Herschel on February 24, 1786. It lies near 110 Virginis and is part of the Herschel 400 Catalogue. It is a member of the NGC 5846 Group of galaxies, itself one of the Virgo III Groups strung out to the east of the Virgo Supercluster of galaxies.