Cooling flow

The cooling flow (literally "cooling flow") exists within the framework of the theory that the intracluster medium  ICM (a plasma cloud) in the center of galaxy clusters cools down, contracts and allows gas to flow in from the outside, thousands of solar masses per year.

The cooling is done by the emission of X-ray - bremsstrahlung whose luminosity is proportional to the square of the density is. Since the density typically increases steeply towards the center of galaxy clusters, the gas temperature in the center is lower than in the outer regions, typically only a third or half, due to the radiation. Although the compression releases heat , the theoretical time interval for the ICM to cool is relatively short, less than a billion years.

calculation

In a stationary system , the loss mass or the degree of cooling of the plasma is obtained from the formula

${\displaystyle {\dot {M}}={\frac {2}{5}}~\mathrm {\frac {L\cdot \mu m}{k\cdot T}} }$

is there

• L the bolometric , d. H. Integrated luminosity of the cooling region across the entire spectrum
• μm is the average molecular mass
• k is the Boltzmann constant
• T is the absolute temperature of the cooling region.

Comparison with the observations

At the moment one has to assume that the enormously high cooling rates to be expected on the basis of the theory are in reality much smaller, as there are so far only few indications for the existence of such cold radiating gases in most galaxy clusters. This is known as the cooling flow problem . There are several theories why so far so little evidence has been found. Some of them are:

• Warming by the active galactic nucleus (also Active Galactic Nucleus , AGN) in galaxy clusters, possibly by sound waves (as can be observed e.g. in the Perseus galaxy cluster or the Virgo galaxy cluster )
• Cosmic Radiation Warming
• Heat conduction from the outside of the heap
• Disappearance of cold gases through absorption of matter
• Mixing of cold with warmer matter.

The warming by the active galactic nuclei is the most common explanation because they emit large amounts of energy during their lifespan and because some of the alternatives listed already have problems in their own theories.

Videos

• What is a cooling flow? from the alpha-Centauri television series(approx. 15 minutes). First broadcast on Jan 19, 2005.

Individual evidence

1. ^ AC Fabian: Cooling Flows in Clusters of Galaxies . In: Annual Review of Astronomy and Astrophysics . tape 32 , no. 1 , September 1994, pp. 277-318 , doi : 10.1146 / annurev.aa.32.090194.001425 ( nedwww.ipac.caltech.edu [accessed April 24, 2010]).
2. ↑ What is a cooling flow? from the alpha-Centauri television series (approx. 15 minutes). First broadcast on Jan. 19, 2005. (Detailed film with explanation of the cooling flow as part of the alpha-centauri series).
3. ↑ JR Peterson, SM Kahn, FBS Paerels, JS Kaastra, T. Tamura, JAM Bleeker, C. Ferrigno, JG Jernigan: High-Resolution X-Ray Spectroscopic Constraints on Cooling-Flow Models for Clusters of Galaxies . In: The Astrophysical Journal . tape 590 , no. 1 , 2003, p. 207-224 , doi : 10.1086 / 374830 .
4. ^ JR Peterson, AC Fabian: X-ray spectroscopy of cooling clusters . In: Physics Reports . tape 427 , no. 1 , 2006, p. 1-39 , doi : 10.1016 / j.physrep.2005.12.007 .