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The Baryon Catastrophe and the multiphase intracluster medium

We review the theories and observations which together have led to the concept of the Baryon Catastrophe: observations of the baryon fraction on the scale of clusters of galaxies appear to be at least three times as high as the universal baryon fraction predicted by the theory of primordial nucleosynthesis in a flat, $Ω_0 = 1$, universe. We investigate whether this discrepancy could be eliminated by treating the intracluster gas as a multiphase medium, and find that this treatment both lowers the calculated mass of gas in a cluster and increases the inferred gravitational potential. These combined effects can reduce the calculated baryon fraction by between a quarter and a half: the precise amount depends upon the volume fraction distribution of density phases in the gas but is independent of the temperature profile across the cluster. Thus moving to a multiphase intracluster medium cannot resolve the Baryon Catastrophe by itself; other possible causes and explanations are discussed.