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Nearby clusters of galaxies. (a) Virgo.This is a rich cluster, with a few thousand members, but it is not very strongly concentrated towards the center. (b) Coma Berenices.This cluster contains more than 1000 galaxies, with a large number of types E and S0.

(Continued) (c) Hercules.This is a small irregular cluster at a distance of 120 Mpc. (d) Centaurus, in the southern hemisphere. [NOAO/AURA/NSF]

(Continued) (c) Hercules.This is a small irregular cluster at a distance of 120 Mpc. (d) Centaurus, in the southern hemisphere. [NOAO/AURA/NSF]

When we add up all the mass that we can see in the cluster, we find that it does not add up to the amount required by the virial theorem. This was originally done by using just the mass of the luminous matter in the galaxies. However, we saw in Chapter 17 that the halos of galaxies may contain dark matter. Even if we don't know what that dark matter is, we know it is there, and can add its mass to that of the luminous matter in each galaxy. However, clusters have many ellipticals and S0 galaxies which may not have massive

Motion of Galaxy within Cluster

Net Motion of Galaxy

Motion of Galaxy within Cluster

Net Motion of Galaxy

Motions of a galaxy in a cluster.The blue arrow shows the overall motion of the cluster.The green arrows show the motions of the galaxies within the cluster.The net motion for each galaxy (shown in red) is the vector sum of its internal motion with the overall motion of the cluster.

Motions of a galaxy in a cluster.The blue arrow shows the overall motion of the cluster.The green arrows show the motions of the galaxies within the cluster.The net motion for each galaxy (shown in red) is the vector sum of its internal motion with the overall motion of the cluster.

halos. We should only add the dark matter that we know is there, so we only add enough to account for the observed rotation curves in different types of galaxies. (We suspect that there may be more dark matter beyond the points where the rotation curves have been measured, because there is no evidence of the rotational velocities beginning to fall off.) Even this amount of dark matter is not enough to account for the virial mass.

Some of the mass may be in the form of low density gas within the cluster, but between the galaxies. This gas has either been ejected from the galaxies, or has fallen into the cluster. In either case, we would expect this gas to be very hot, about 107 K. It should be hot enough to give off faint X-ray emission. In fact, such emission is observed. In Figs. 18.4 and 18.5, we see X-ray images of two clusters. The hot gas contributes a significant amount of mass, but doesn't completely solve the problem.

There are still two possible solutions. One is that the individual galaxies have, as some have suspected, halos that go out even farther than the rotation curves can be measured. There is evidence to support this in studies of the interactions of binary galaxies. The advantage of a binary galaxy over the rotation curve studies is that the galaxies in a binary system are far enough apart to sample the full mass of each other. The other possibility is that the clusters contain their own dark matter. This matter may

Virgo Cluster

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