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3.6. Cluster Baryon and Dark Matter Masses

This important probe was pioneered by White et al. (1993). In the survey by White & Fabian (1995) the ratio of the mass in X-ray emitting gas to the gravitational mass in rich clusters of galaxies is

Equation 20 (20)

Myers et al. (1997) find from the measurement of the Sunyaev-Zeldovich effect in three clusters

Equation 21 (21)

In their contributions to this discussion Silk and Turner explain why the consensus value of the density parameter in baryons to account for light element abundances is Omegabaryon = 0.02 / h2. If clusters are fair samples of baryon and total masses then Omegabaryon / Omegam is the same as (Mbaryon / Mgrav)cl. If most of the cluster baryons are in the plasma we get from these mass ratios Omegam = (0.36 ± 0.09)h-1/2 and Omegam = (0.33 ± 0.06)h-1. The correction for baryons in stars decreases Omegam. Energy injected by winds from supernovae in cluster galaxies would tend to lower the plasma mass; a correction for this effect would further lower Omegam (Metzler & Evrard 1998). At h > 0.5 this measure of Omegam is well below Einstein-de Sitter.

On the other hand, if baryons settled to cluster centers, increasing the local ratio of baryon to total mass, it would bias this measure of Omegam low. It is not hard to make up a story for how this might have happened. Imagine that before there were clusters there were gas clouds dense enough that the baryons dissipatively settled, leaving dark matter halos. We have to postulate the clouds were small enough that the radiation from this dissipative settling is not objectionably hard, and we have to postulate that feedback from star formation prevented catastrophic collapse of the baryons. Now imagine many of these systems fall together to form a proto-cluster. Numerical N-body simulations of merging show that the dense parts of the substructure tend to settle relative to less dense parts, producing the wanted segregation of baryons from the dark matter. Numerical simulations of cluster formation fail to show any evidence of this story; I do not know whether that is because it is only a story or possibly because it is hard to explore all scenarios in numerical simulations.

One does hears mention of the possibility of an inhomogeneous primeval entropy per baryon, but with little enthusiasm.

As indicated in line 2d this constraint on Omegam so far has proved difficult to finesse.

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