Intracluster medium
As intracluster medium ( ICM , literally medium within the pile ) is referred to in the astronomy the thin hot gas the space between the galaxies in galactic clusters filled.
The temperatures of the gas, which consists mainly of hydrogen and helium , are between 10 and 100 million Kelvin . The ICM is ionized ( plasma ) and can be observed through its strong emission of X-rays . It represents a form of the intergalactic medium .
warming
The ICM is heated to its extremely high temperatures by gravitational energy, which is released from smaller structures when the galaxy cluster is formed. The kinetic energy of the gravitational field is carried shockwaves in heat converted.
The high temperatures are responsible for the fact that all the elements occurring in the ICM are in ionized form; the atomic nuclei of the lighter elements are ionized to such an extent that they no longer have any electrons .
composition
The ICM consists primarily of ordinary baryonic matter (mainly hydrogen and helium in an ionized state). This plasma is enriched with heavier elements such as B. iron . The amount of heavy elements in relation to hydrogen, the metallicity , is about a third of that of the sun. Most of the baryons in the cluster (80–95%) are in the ICM and not in the luminous matter of galaxies and stars. The vast majority of the mass of a galaxy cluster (around 80%), however, consists of non-baryonic dark matter .
Although the ICM as a whole contains the majority of the baryons in a galaxy cluster, with typical values of around 10 −3 particles per cubic centimeter it is not very dense. The mean free path of a particle is around 10 16 m or one light year .
The strong gravitational field of the clusters also means that they can even hold onto elements that have arisen in high-energy supernovae . Although these escape from the galaxies in which they are generated, they remain trapped in the cluster and enrich its ICM. Observations of the composition of the ICM in clusters with different cosmological redshifts (which corresponds to an observation of different times in the past) can therefore provide insights into the history of the formation of the elements in the universe ( nucleosynthesis ).
observation
Due to its high temperature, the ICM primarily emits X-rays. It is generated as bremsstrahlung when electrons are deflected by ions and as X-ray emission lines from the heavy elements. The luminosity of the gas is proportional to the square of the gas density and the root of the temperature.
X-ray telescopes can measure the X-ray spectrum of the ICM radiation and map the distribution of their X-ray brightness. The gas temperature and the metallicity of the ICM are obtained from the X-ray spectrum, and the distribution of the gas density is obtained from the brightness distribution. Assuming that the gas is in hydrostatic equilibrium , the total mass of the galaxy cluster can be determined from the temperature and the density distribution.
The density of the ICM increases sharply towards the center of the galaxy cluster. At the same time, the temperature of this central region is usually only between half and a third of the values measured in the outer areas. The metallicity of the gas also increases from the outside towards the center. In some galaxy clusters (e.g. in the Centaurus cluster ) it reaches values above those of our sun.
Another possibility for observation results from the Sunyaev-Zeldovich effect .
Cooling
Due to the high density of the ICM in the core of galaxy clusters, large amounts of X-rays are emitted. If there is no additional heating, the ICM should actually cool down, be further compressed in the center and finally converted into stars. As a result of the compression, a continuous flow of matter should form from the outer areas of the galaxy cluster into its core area, a cooling flow .
Observations with the X-ray telescopes Chandra and XMM-Newton at the beginning of the 21st century, however, showed that the temperature in cluster centers does not drop as much as the cooling-flow scenario , which goes back to Andrew Fabian , requires. In addition, the galaxies in the cluster center show no signs of the suspected strong star formation . To solve this cooling-flow problem , a number of approaches are discussed, including e.g. B. the heating of the gas by active galaxy nuclei .
Individual evidence
- ↑ Loewenstein, Michael. Chemical Composition of the Intracluster Medium , Carnegie Observatories Centennial Symposia, p.422, 2004.