A.C. Fabian – författare

Gas at temperatures exceeding one million degrees is common in the Universe. Indeed it is likely that most of the gas in the Universe exists in intergalactic space in this form. Such highly-ionized gas, or plasma, is not restricted to the rarefied densities of intergalactic space, but is also found in clusters of galaxies, in galaxies themselves, in the expanding remnants of exploded stars and at higher densities in stars and the collapsed remains of stars up to the highest densities known, which occur in neutron stars. The abundant lower-Z elements, at least, in such gas are completely ionized and the gas acts as a highly conducting plasma. It is therefore subject to many cooperative phenomena, which are often complicated and ill-understood. Many of these processes are, however, well-studied (if not so well-understood) in laboratory plasmas and in the near environment of the Earth. Astronomers therefore have much to learn from plasma physicists working on laboratory and space plasmas and the parameter range studied by the plasma physicists might in turn be broadened by contact with astronomers. With that in mind, a NATO Advanced Research Workshop on Physical Processes in Hot Cosmic Plasmas was organized and took place in the Eolian Hotel, Vulcano, Italy on May 29 to June 2 1989. This book contains the Proceedings of that Workshop.

X-ray astronomers discovered the diffuse gas in clusters of galaxies about 20 years ago. It was later realized that the central gas density in some clusters, and in elliptical galaxies, is so high that radiative cooling is a significant energy loss. The cooling time of the gas decreases rapidly towards the centre of the cluster or galaxy and is less than a Hubble time within the innermost few hundred kiloparsecs. This results in a cooling flow in which the gas density rises in order to maintain pressure to support the weight of the overlying gas. The rate at which mass is deposited by the flow is inferred to be several hundreds of solar masses per year in some clusters. The fraction of clusters in which cooling flows are found may exceed 50 per cent. Small flows probably occur in most normal elliptical galaxies that are not in rich clusters. The implications of this simple phenomenon are profound, for we appear to be witnessing the ongoing formation of the central galaxy. In particular, since most of the gas is undetected once it cools below about 3 million K, it appears to form dark matter. There is no reason why it should be detectable with current techniques if each cooling proton only recombines once and the matter condenses into objects of low mass.

Clusters and superclusters of galaxies are the largest objects in the Universe. They have been the subject of intense observational studies at a variety of wavelengths, from radio to X-ray which has provoked much theoretical debate and advanced our understanding of the recent evolution of the large-scale structure of the Universe. The current status of the subject is reviewed in this volume by active researchers who lectured at a NATO Advanced Study Institute held in Cambridge, England in July 1991. Much of the material is presented in a pedagogical manner and will appeal to scientists, astronomers and graduate students interested in extragalactic astronomy.

Gas at temperatures exceeding one million degrees is common in the Universe. Indeed it is likely that most of the gas in the Universe exists in intergalactic space in this form. Such highly-ionized gas, or plasma, is not restricted to the rarefied densities of intergalactic space, but is also found in clusters of galaxies, in galaxies themselves, in the expanding remnants of exploded stars and at higher densities in stars and the collapsed remains of stars up to the highest densities known, which occur in neutron stars. The abundant lower-Z elements, at least, in such gas are completely ionized and the gas acts as a highly conducting plasma. It is therefore subject to many cooperative phenomena, which are often complicated and ill-understood. Many of these processes are, however, well-studied (if not so well-understood) in laboratory plasmas and in the near environment of the Earth. Astronomers therefore have much to learn from plasma physicists working on laboratory and space plasmas and the parameter range studied by the plasma physicists might in turn be broadened by contact with astronomers. With that in mind, a NATO Advanced Research Workshop on Physical Processes in Hot Cosmic Plasmas was organized and took place in the Eolian Hotel, Vulcano, Italy on May 29 to June 2 1989. This book contains the Proceedings of that Workshop.

X-ray astronomers discovered the diffuse gas in clusters of galaxies about 20 years ago. It was later realized that the central gas density in some clusters, and in elliptical galaxies, is so high that radiative cooling is a significant energy loss. The cooling time of the gas decreases rapidly towards the centre of the cluster or galaxy and is less than a Hubble time within the innermost few hundred kiloparsecs. This results in a cooling flow in which the gas density rises in order to maintain pressure to support the weight of the overlying gas. The rate at which mass is deposited by the flow is inferred to be several hundreds of solar masses per year in some clusters. The fraction of clusters in which cooling flows are found may exceed 50 per cent. Small flows probably occur in most normal elliptical galaxies that are not in rich clusters. The implications of this simple phenomenon are profound, for we appear to be witnessing the ongoing formation of the central galaxy. In particular, since most of the gas is undetected once it cools below about 3 million K, it appears to form dark matter. There is no reason why it should be detectable with current techniques if each cooling proton only recombines once and the matter condenses into objects of low mass.