Cooling Flow

The so-called cooling flow ( literally " cooling - flow " ) exists within the framework of a theory which states that the intracluster medium or ICM ( plasma cloud) cools down in the center of galaxy clusters, contracts, and can subsequently flow from the outside gas, thousands of solar masses per year. The cooling is done by emitting X-ray Bremsstrahlung whose brightness is proportional to the square of the density. Since the density typically rises steeply toward the center of galaxy clusters, the gas temperature in the center is by the radiation lower than in the outer regions, typically only a third or half. Although the compression releases heat, the theoretical time interval for the cooling of the ICM with less than a billion years is relatively short.

Calculation

In a stationary system, the loss of weight or the degree of cooling of the plasma is obtained by the formula

While L is the bolometric luminosity over the entire range of cooling region, T its temperature, k is the Boltzmann constant and the average molecular mass microns.

Comparison with the observations

Currently, one must assume that the basis of the theory actually to be expected extremely high cooling rates in reality are much smaller, since there are as yet few indications of the existence of such cold radiating gases in most clusters of galaxies. This is referred to as a cooling flow problem. There are several theories as to why we previously found so few clues. Some of these are:

• Heating by the central active galactic nucleus ( Active Galactic Nucleus also AGN) in clusters of galaxies, perhaps by sound waves ( such as can be observed, for example in the Perseus cluster of galaxies or the Virgo cluster of galaxies )
• Heat conduction from the outer regions of the cluster
• Heating by cosmic rays
• Disappearance of the cold gases by absorbing matter
• Mixing of cold matter with warmer

The heating by the active galactic nuclei is the most common explanation, because these during their lifetime emit large amounts of energy, and because some of the listed alternatives already have problems even in their theory.

Videos

• What is a Cooling Flow? from the TV - series alpha Centauri (about 15 minutes). First aired on 19 Jan. 2005.