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Luminous Matter Dark Matter

ROSAT X-ray image of the Virgo cluster (Fig. 18.2a) shows the hot intracluster, intergalactic gas. [NASA/MPI]

Chandra X-ray image of the Centaurus cluster (Fig. 18.2d), showing more detail in the hot gas [NASA].

Chandra X-ray image of the Centaurus cluster (Fig. 18.2d), showing more detail in the hot gas [NASA].

Possible distribution of dark matter in a cluster of galaxies. Each blue patch indicates the position of luminous matter within the cluster.The red areas indicate the locations of the dark matter.The darker the areas are, the greater the concentration of dark matter.

be the same as that in the halos of galaxies, but there just may be additional amounts in the cluster, not bound to any one galaxy. If a rich cluster has the mass implied by the virial theorem, then the mass-to-light ratio is about 200. This would be consistent with either the extended halos in individual galaxies or the generally distributed dark matter. A possible distribution of dark matter in a cluster of galaxies is shown schematically in Fig. 18.6.

Another interesting feature about clusters of galaxies is that giant elliptical galaxies are found near the centers of some clusters (such as M87, in Fig. 17.2a). These galaxies are central dominant or cD galaxies. Some cD galaxies also seem to have multiple nuclei. It has been noted that the center of a cluster is the most likely place for galaxies to pass near each other. Some galaxy collisions result in galaxy mergers. Once a few galaxies have merged, they can swallow galaxies that pass too close. The process is called galactic cannibalism.

The whole subject of galaxy encounters is under active study. Numerical simulations have been carried out to find out what happens to the stars and gas in each of the two colliding galaxies, both for very close encounters and for direct collisions. The result of one such calculation is shown in Fig. 18.7. Some examples of interacting galaxies are shown in Fig. 18.8. As you can see, the calculations produce results that look like objects that are actually observed.

Interacting galaxies. Steps in the computer simulation. (a) The galaxies are far apart. (b) They are closer together, and the effects of the interaction are showing. Note that the tidal effects, tending to stretch the structures, are very important. In encounters, individual stars never actually touch. [Visualization by Frank Summers (STScI); simulation by Chris Mihos (Case Western Reserve University) and Lars Hemquist (Harvard University)]

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