Evidence for dark matter

9.12.1 Evidence for dark matter

In Chapter 8 it has been described how one of the fi rst pieces of evidence for the presence of dark matter emerged – dark matter being needed to explain the anoma- lous rotation curves of galaxies. In this section further evidence, some very recent, will be discussed.

Cluster dynamics

The fi rst evidence of a large amount of unseen matter came from observations made by Fritz Zwicky in the 1930s. He studied the Coma cluster of galaxies, 321 million light years distant, and observed that the outer members of the

cluster were moving at far higher speeds than were expected. Suppose a clus- ter of galaxies was created which was not in motion. Gravity would cause it to collapse down into a single giant body. If, on the other hand, the galaxies were initially given very high speeds relative one to another, their kinetic energy would enable them to disperse into the universe and the cluster would disperse, just as a rocket travelling at a suffi ciently high speed could escape the gravitational fi eld of the Earth. The fact that we observe a cluster of galaxies many billions of years after it was created implies that there must be an equilibrium balancing the gravitational pull of the cluster’s total mass and the average kinetic energy of its members. This concept is enshrined in what is called the virial theorem so that, if the speeds of the cluster members can be found, it is possible to estimate the total mass of the cluster. Zwicky carried out these calculations and showed that the Coma cluster must contain signifi cantly more mass than could be accounted for by its visible content.

Introduction to Astronomy and Cosmology

Figure 9.11 NASA’a Chandra X-ray satellite image of hot gas surrounding the galaxy NGC 4555. Image: NASA/CXC/E.O’Sullivan et al.

Gas entrapment

NASA’s Chandra X-ray Observatory has revealed that the elliptical galaxy NGC 4555 is embedded in a cloud of gas having a diameter of about 400 000 light years and a temperature of 10 million°C (Figure 9.11). At this temperature the molecules of gas would be travelling at very high speeds and the mass of the stars within the galaxy would be far too low to prevent its escape. For the gas to remain in the vicinity of the galaxy the total mass of the system must be about 10 times the combined mass of the stars in the galaxy and about 300 times that of the gas cloud.

Gravitational lensing

Earlier, the formation of multiple images of a distant object by a foreground galaxy has been discussed. On a much larger scale the mass of a cluster of galaxies can distort the images of more distant objects. The image of the Abell 2218 cluster is a wonderful example showing images of more distant galaxies that have been distorted into arcs (Figure 9.12). The amount of distortion will be a function of the total mass of the intervening cluster, so this gives a way of estimating the total mass of galaxy clusters, confi rming the existence of dark matter. Using this technique, astronomers have even shown how the distribution of dark matter has

Cosmology – the Origin and Evolution of the Universe

Figure 9.12 The Abell 2218 cluster imaged by the Hubble Space Telescope. Image: NASA, A. Fruchter and the ERO team, STScI.