The Net Advance of Physics: The Nature of Dark Matter, by Kim Griest -- Section 2B.

Clusters of Galaxies

Moving to larger scales, the methods of determining become less

secure, but give larger values. There is a great deal of new evidence

on dark matter in clusters of galaxies, coming from gravitational

lensing [7], from X-ray gas temperatures [8], and from the motions

of cluster member galaxies. For example, consider the Coma cluster

which contains around a thousand galaxies. White et al. [9] recently

collated some of the data on the Coma cluster, reporting separate

measurements of the amount of mass in stars, hot gas, and in total.

Within a radius of 1.5 Mpc, they give

where the total mass is estimated in two completely different ways.

The first method is a refinement of Zwicky's method of using the

radial velocities of the member galaxies, and the assumption of

virialization to gauge the depth of the gravitational potential well.

The second method makes use of the ROSAT X-ray maps and the

assumption of a constant temperature equilibrium to get the same

information. Remarkably the two methods give the same mass

within errors. Thus with a mass-to-light ratio of

, one finds , if the inner 1.5 Mpc

of Coma is representative of the Universe as a whole.

There is, however, something disconcerting about the above

numbers. As pointed out by White, et al. [9]

Now the Coma cluster is large enough that one might expect its

baryon to dark matter ratio to be the Universal value,

( ), and in fact White et al. argue

that this is the case. Then the inequality above should apply to the

entire Universe. But, big bang nucleosynthesis limits

. If , the two inequalities are in quite

strong disagreement for any value of h. So this is a puzzle. The

conclusion of White et al. is that either is not unity, or that big

bang nucleosynthesis is not working. However, there are other

possible explanations, notably that the measurements of the total

mass in clusters by gravitational lensing tend to give larger total

mass than the X-ray and virial methods, and that mass and velocity

bias may mean that clusters are not representative of the Universe

as a whole. The story is clearly not yet finished.

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