Cooling flow
A cooling flow [1] is the theory that the Intracluster medium 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 density of the plasma steeply rises towards the centre of many galaxy clusters and the temperature typically drops to a half or a third of the outer temperature. As the brightness of X-ray emission of the plasma is proportional to the square of the density, the plasma should rapidly lose energy by X-ray emission. The typical timescale for the Intracluster medium to cool is relatively short, less than a billion years.
It is currently thought that the very large amounts of expected cooling are in reality much smaller, as there is little evidence for cool X-ray emitting gas in many of these systems [2]. Theories for why there is little evidence of heating include[3]
- Heating by the central Active galactic nucleus (AGN) in clusters, possibly via sound waves
- Thermal conduction of heat from the outer parts of clusters
- Cosmic ray heating
- Hiding cool gas by absorbing material
- Mixing of cool gas with hotter material
Heating by AGN is the most popular explanation, as they emit a lot of energy over their lifetimes, and some of the alternatives listed have theoretical problems.
References
- ^ Fabian A.C.: Cooling flows in clusters of galaxies, Annu. Rev. Astron. Astrophys. 1994. 32: 277-318
- ^ Peterson J.R., et al.: High-Resolution X-ray Spectroscopic Constrains on Cooling Flow Models for Clusters of Galaxies. ApJ 2003. 590: 207
- ^ Peterson J.R., Fabian A.C.: X-ray spectoscopy of cooling clusters, Physics Reports. 2006. 427: 1