Annu. Rev. Astron. Astrophys. 1994. 32: 531-590
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2.5. Groups and Clusters of Galaxies

Galaxies are clumped on various scales (as members of binaries, small groups, and rich clusters) and velocity dispersion measurements indicate that the dynamical mass exceeds the visible mass on all these scales. Binaries can only be studied statistically (because one does not know the orbital inclination in any individual case), so the data are less clear-cut here; however, there is compelling evidence for dark mass in clusters of galaxies. This is confirmed by X-ray data on the gas temperature (which provide an independent measure of the gravitational potential). In rich clusters, the dark mass dominates by at least a factor of 10 and the recent discovery of hot gas in two small groups of galaxies (HCG92 and HCG62) by ROSAT shows that there are comparable amounts of dark matter there (Mulchaey et al 1993, Ponman & Bertram 1993).

In assessing whether the dark mass in groups and clusters can be baryonic, it is important to determine whether it is the same as the halo dark matter. Although the cluster dark mass cannot all be associated with individual galaxies now - for then dynamical friction would result in the most massive galaxies being dragged into the cluster center (White 1976) - it may still have derived from the galaxies originally. Indeed, in the hierarchial clustering picture one would expect the galaxies inside a cluster to be stripped of their individual halos to form a collective halo (White & Rees 1978). However, this would only suffice to explain all the cluster dark matter if the original galactic halos were larger than about 200 kpc and, in this case, we will see that they could not be purely baryonic unless one invokes inhomogeneous Big Bang nucleosynthesis.