Cooling flow
A cooling flow occurs when the intracluster medium (ICM) in the centres of galaxy clusters should be rapidly cooling at the rate of tens to thousands of solar masses per year.[1] This should happen as the ICM (a plasma) is quickly losing its energy by the emission of X-rays. The X-ray brightness of the ICM is proportional to the square of its density, which rises steeply towards the centres of many clusters. Also the temperature falls to typically a third or a half of the temperature in the outskirts of the cluster. The typical [predicted] timescale for the ICM to cool is relatively short, less than a billion years. As material in the centre of the cluster cools out, the pressure of the overlying ICM should cause more material to flow inwards (the cooling flow).
In a steady state, the rate of mass deposition, i.e. the rate at which the plasma cools, is given by
where L is the bolometric (i.e. over the entire spectrum) luminosity of the cooling region, T is its temperature, k is the Boltzmann constant and μm is the mean molecular mass.
Cooling flow problem
[edit]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] This is the cooling flow problem. Theories for why there is little evidence of cooling include[3]
- Heating by the central Active galactic nucleus (AGN) in clusters, possibly via sound waves (seen in the Perseus and Virgo clusters)
- 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
[edit]- ^ Fabian, A. C. (1994). "Cooling flows in clusters of galaxies". Annu. Rev. Astron. Astrophys. 32: 277–318. Bibcode:1994ARA&A..32..277F. doi:10.1146/annurev.aa.32.090194.001425.
- ^ Peterson, J. R.; Kahn, S. M.; Paerels, F. B. S.; Kaastra, J. S.; Tamura, T.; Bleeker, J. A. M.; Ferrigno, C.; Jernigan, J. G. (2003-06-10). "High-Resolution X-Ray Spectroscopic Constraints on Cooling-Flow Models for Clusters of Galaxies". The Astrophysical Journal. 590 (1): 207–224. arXiv:astro-ph/0210662. Bibcode:2003ApJ...590..207P. doi:10.1086/374830. ISSN 0004-637X. S2CID 18000290.
- ^ Peterson, J.R.; Fabian, A.C. (2006). "X-ray spectroscopy of cooling clusters". Physics Reports. 427 (1): 1–39. arXiv:astro-ph/0512549. Bibcode:2006PhR...427....1P. doi:10.1016/j.physrep.2005.12.007. ISSN 0370-1573. S2CID 11711221.
Further reading
[edit]- Qin, Bo; Wu, Xiang-Ping (2001-07-19). "Constraints on the Interaction between Dark Matter and Baryons from Cooling Flow Clusters". Physical Review Letters. 87 (6): 061301. arXiv:astro-ph/0106458. Bibcode:2001PhRvL..87f1301Q. doi:10.1103/physrevlett.87.061301. ISSN 0031-9007. PMID 11497819. S2CID 13510283.
- Chuzhoy, Leonid; Nusser, Adi (2006-07-10). "Consequences of Short-Range Interactions between Dark Matter and Protons in Galaxy Clusters". The Astrophysical Journal. 645 (2): 950–954. arXiv:astro-ph/0408184. Bibcode:2006ApJ...645..950C. doi:10.1086/504505. ISSN 0004-637X. S2CID 16131656.
- 5.7. Cooling flows and accretion by cDs (in X-ray Emission from Clusters of Galaxies. Sarazin 1988)