Comparison of the Chandra image of the X-ray emission from the intracluster medium in the core of the Abell 2199 galaxy cluster against the optical emission of the galaxies (from the DSS)

In astronomy, the intracluster medium (or ICM) is the superheated gas present at the center of a galaxy cluster. The Chandra X-ray Observatory is a Satellite launched on STS-93 by NASA on July 23, 1999. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. The Digitized Sky Survey (DSS is a Digital version of several photographic Atlases of the night sky and an ongoing project to produce more digital versions Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation This plasma is heated to temperatures of between roughly 10 and 100 million Kelvin and consists mainly of ionised hydrogen and helium, containing most of the baryonic material in the cluster. In Physics and Chemistry, plasma is an Ionized Gas, in which a certain proportion of Electrons are free rather than being bound The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical Baryons are the family of Subatomic particles with a Baryon number of 1 The ICM strongly emits X-ray radiation. X-radiation (composed of X-rays) is a form of Electromagnetic radiation.

Heating

The ICM is heated to high temperatures by the gravitational energy released by the formation of the cluster from smaller structures. Gravitational energy is the energy associated with the gravitational field Kinetic energy gained from the gravitational field is converted to thermal energy by shocks. The kinetic energy of an object is the extra Energy which it possesses due to its motion A gravitational field is a model used within Physics to explain how gravity exists in the universe Thermal energy is the sum of the sensible energy and latent energy. For the music album by Converter see Shock Front For the 1977 horror film see Shock Waves A shock wave (also called The high temperature ensures that the elements present in the ICM are ionised. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge Light elements in the ICM have all the electrons removed from their nuclei. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J

Composition

The ICM is composed primarily of ordinary baryons (mainly ionised hydrogen and helium). Baryons are the family of Subatomic particles with a Baryon number of 1 This plasma is enriched with heavy elements, such as iron. Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 The amount of heavy elements relative to hydrogen (known as metallicity in astronomy) is roughly a third of the value in the sun. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen The Sun (Sol is the Star at the center of the Solar System. Most of the baryons in the cluster (80-95%) reside in the ICM, rather than in the luminous matter, such as galaxies and stars. However, most of the mass in a galaxy cluster consists of dark matter. In Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from

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. In Physics the mean free path of a particle is the average distance covered by a particle ( Photon, Atom or Molecule) between subsequent impacts

The strong gravitational field of clusters means that they can retain even elements created in high-energy supernovae. A supernova (plural supernovae or supernovas) is a stellar Explosion. Studying the composition of the ICM at varying redshift (which results in looking at different points back in time) can therefore give a record of element production in the universe if they are typical^[1]. In Physics and Astronomy, redshift occurs when Electromagnetic radiation – usually Visible light – emitted or reflected by

Observing

As the ICM is so hot, it mostly emits X-ray radiation by the bremsstrahlung process and X-ray emission lines from the heavy elements. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. Bremsstrahlung ( pronounced, from German de ''bremsen'' "to brake" and de ''Strahlung'' "radiation" i A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared These X-rays can be observed using an X-ray telescope. X-ray astronomy is an observational branch of Astronomy, which deals with the study of X-ray emission from celestial objects Depending on the telescope, maps of the ICM can be made (the X-ray emission is proportional to the density of the ICM squared), and X-ray spectra can be obtained. A spectrum (plural spectra or spectrums) is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The brightness of the X-rays tells us about the density of the gas. The spectra allow temperature and metallicity of the ICM to be measured.

The density of the ICM rises towards the centre of the cluster with a 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. The metallicity rises from the outer region towards the centre. In some clusters (e. g. the Centaurus cluster) the metallicity of the gas can rise above that of the sun. The Centaurus Cluster (A3526 is a cluster of hundreds of galaxies, located approximately 155 million Light years away in the Centaurus

Cooling flow

As the ICM in the core of many galaxy clusters is dense, it emits a lot of X-ray radiation (the emission is proportional to the density-squared). In the absence of heating, the ICM should be cooling. As it cools, hotter gas will flow in to replace it. This is known as a cooling flow. A cooling flow occurs according to the theory that the Intracluster medium (ICM in the centres of Galaxy clusters should be rapidly cooling at the rate of tens to thousands The cooling flow problem is the lack of evidence of cooling of the ICM.

Intracluster dust

It is believed, that as part of its cold component, the inhomogeneous ICM contains dust particles in its densest parts, which can survive the strong X-ray and UV-radiation due to self-shielding. The presence of dust cause extiction and reddening at the visual wavelengths, and excess emission in the infrared. The strength and spectra of this excess infrared emission depends on the amount and temperature of the intergalactic dust; the latter is assumed to be in the range of T_icd=10. . . 50K. The amount of expected reddening due to intracluster dust is E(B-V) < 0. 01 mag, therefore it is very difficult to observe, and has just been detected using Sloan Digital Sky Survey data ^[2]

The infrared emission of intracluster dust is important for the detection of the cosmic infrared background as well, since the intracluster dust in our Local Group could be an important foreground component. The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic Redshift survey using a dedicated 2 History Recognizing the cosmological importance of the darkness of the night sky (the so-called Olbers paradox) and the first speculations on an Extragalactic background The Local Group is the group of galaxies that includes our galaxy the Milky Way. The first attempt to detect intracluster dust by its thermal emission was made by the ISOPHOT instrument of the Infrared Space Observatory ^[3]. The Infrared Space Observatory ( ISO) was a Space telescope for Infrared light designed and operated by the European Space Agency (ESA This study gave upper limits for the amount of intracluster dust in several Abell clusters, and detected a small amount of dust in the Coma cluster of M_D=10⁷M_sun. The Abell catalog of rich clusters of galaxies is an all-sky catalog of 4073 rich Galaxy clusters of nominal Redshift z = 0 Even the latest upper limits by the Spitzer Space Telescope ^[4] indicate, that the amount of intracluster dust in the galaxy cluster Abell 2029 - and the extrapolated value to our Local Group - is too low to cause a notable contribution to the cosmic infrared background. The Spitzer Space Telescope (formerly the Space Infrared Telescope Facility, SIRTF) is an Infrared Space observatory. Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation Abell 2029 is a large cluster of galaxies a billion light years away in the constellation Serpens, close to its boundary with Virgo. History Recognizing the cosmological importance of the darkness of the night sky (the so-called Olbers paradox) and the first speculations on an Extragalactic background

See also

• galaxy cluster
• intergalactic medium

References