|Annu. Rev. Astron. Astrophys. 1994. 32:
Copyright © 1994 by . All rights reserved
The X-ray observations of cooling flows form a consistent picture showing that much of the gas surrounding the central galaxy in a typical cluster is cooling and slumping inward. Inhomogeneities in the gas lead to spatially distributed mass deposition such that the cooled gas is widely spread throughout the flow. X-ray absorption is evidence for the cooled material, much of which must remain at radii of 100 kpc and more from the central galaxy. Observations at other wavebands support the high pressures derived from the X-ray data and clearly show anomalies in the central few kpc that are peculiar to these objects. There is no observational support from these wavebands for the absorbing or cooled matter, which must accumulate in the form of dense clouds, low-mass stars, or dust grains. The nature of embedded gas clouds is restricted by tight observational limits on HI and CO. There is, of course, much evidence for dark matter in these regions, so the total accumulated mass is no problem.
The behavior of a cooling flow requires that conduction is highly suppressed and that gas clouds retain some coherence. This requires tangled magnetic fields which complicate the geometry and evolution of cooled gas clouds. The gas is probably turbulent, especially in the center, and its overall state is complex. The ICM may be at least as complicated as our own interstellar medium.
There is growing evidence from observations of the most powerful distant radio objects such as the radio galaxies and quasars that cooling flows power, provoke, and shape luminous radio sources from the central galaxy. The distributed hot and cold gas in a surrounding cooling flow explains many of the phenomena associated with these objects. Finally, the hierarchical formation of structure in the Universe means that strong cooling flows were common in the past and that the processes involved in them have shaped the upper limit to the luminosity function of galaxies.
I am very grateful to Carolin Crawford and Claude Canizares for help and discussions. I also thank Steve Allen, Hans Böhringer, Alastair Edge, Keith Gendreau, Roderick Johnstone, Paul Nulsen, Peter Tribble, and David White for comments and help with the preparation of figures, and Professor Y. Tanaka and his colleagues at ISAS for hospitality while much of this review was written. The Royal Society is thanked for support.