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The hot gas has a very long cooling time

The hot gas has a very long cooling time - (of course...

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The hot gas has a very long cooling time (of order the Hubble time) except in denser areas, which can produce a cooling flow onto individual galaxies or in cluster cores. This happens because of the interesting behavior of the cooling rate for hydrogen-dominated plasmas as a function of temperature and density; since brehmsstrahlung is important, the n e 2 dependence can pull the rug out from under a pressure-supported gas. That is, for a purely cooling plasma, the time it spends at a given temperature is inversely related to the efficiency of cooling at that temperature, so that it drops rapidly through temperature regimes where cooling is strong (such as through optical and UV line emission) and slowly through regimes with very weak cooling, such as 10 7 K at low density. Cooling flows have been suggested as a way to build central cluster galaxies over cosmic time, but (with a few notable exceptions such as Abell 1795) there's no trace of the massive star formation expected if the gas is being dumped into the center. There are
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Unformatted text preview: (of course) models in which the stars are all very low mass and conveniently unobservable, but the role of nonradial flow and magnetic fields may well defeat those last few kiloparsecs. It has also been wdely suggested that AGN outbursts heat the IC sufficiently to dafeat cooling flows; evidence includes finding places where the radio plasma from double sources has shoved the hot gas around, and waves indicating kinetic-energy input. Cluster X-ray emission is perhaps a better tracer of dynamical evolution than galaxy counts, since it is smooth and not limited by galaxy statistics. This is also the technique of choice for finding distant clusters (as extended X-ray sources) since confusion at faint magnitudes makes optical identification difficult and unreliable. Note that the total mass of gas usually exceeds that in stars for a typical cluster....
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