Annu. Rev. Astron. Astrophys. 1994. 32: 277-318
Copyright © 1994 by . All rights reserved

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4.3 Summary of the Conditions in a Cooling Flow

The picture that we have assembled of the inner ICM in a cluster with a cooling flow is tentative. The gas is inhomogeneous when hot, with densities ranging over at least a factor of 2 (and temperature ranging in the opposite sense to maintain pressure equilibrium). Conduction is highly suppressed and the cooler, denser clouds cool out of the flow at the largest radii, condensing into very dense blobs, and the hotter, more tenuous gas survives to the core of the flow. Weak magnetic fields bind the clouds together against the destructive ram-pressure forces and so determine the mass range of surviving clouds. As a cloud cools, its cooling time rapidly reduces to less than a million years and it drops out of pressure and ionization equilibrium. Depending on the geometry, the magnetic field is amplified and dominates the pressure and so supports the cloud against further compression. Cooling continues in the gas and slowly the magnetic field may be expelled from the cold core of the cloud by ambipolar diffusion. The gas temperature then drops to very low values as the gas becomes increasingly molecular and possibly dusty. If the cloud core is above the Jeans mass it may collapse and produce low-mass stars or brown dwarfs. This is the likely fate for most of the cooled gas, which left the flow at large radii. The efficiency of any star formation must be low in order that the X-ray absorbing column density remains. In the core of the flow, however, it is probable that a cloud collides with another one before forming this stage, leading to strong detectable optical line emission and more ionized, massive magnetic clouds. If above the critical mass, these may form some massive stars and possibly globular clusters. Small dusty shreds of cold gas and clouds may scatter the blue continuum from an active nucleus.