Cluster of Galaxies

The first evidence that there was a decent amount of significant matter that we could not see was in clusters of galaxies. These are simply aggregates of a few hundred to a few thousand clusters otherwise isolated in space.

In the 1930’s two relatively close clusters were examined. The Coma cluster and the Virgo cluster. Since clusters are made up of individual galaxies, they looked into each cluster. What they found was that the velocities of the galaxies were very different to what they expected. What did this mean?

Before answering that, we must understand that in a group of galaxies the only important force is gravitation. It is the pulling of the galaxies on each other that gives rise to their velocities. The velocities can indicate the total mass inside the cluster in two ways. The first way is rather simple: The more collective mass inside the cluster, the greater the forces acting on each galaxy. Which in turn accelerates the galaxies to higher velocities.

The velocities can indicate the total mass inside the cluster in two ways. The first way is simple; the more mass in the cluster, the greater the forces acting on each galaxy, which accelerates the galaxies to higher velocities.

The second way that velocities indicate the mass in the cluster is also rather simple. If the velocity of a given galaxy is too large, it will be able to break free of the gravitational pull of the cluster. Just like in a escape velocity. That is if the galaxy velocities are larger than the escape velocity of the cluster. This means the galaxy will simply leave the cluster. Hence, by knowing that all galaxies have velocities less than the escape velocity, one can estimate the total mass.

Looking back this was fairly strong evidence; still it was not treated as such. The also lies a problem here. When doing observations such as this, there are many ways these can go wrong; Contamination’.

When we are observing something as vast as a cluster of galaxies, even though the velocities are quite large, they are negligible when compared to the vastness of the cluster it lies within. For that matter, even observing a cluster over years, does not show any significant change. One cannot really see them running around. Hence, it can mean that a galaxy with a high speed is leaving the cluster, or may have never been part of that cluster, or maybe just passing through, or maybe it was only a galaxy in the foreground ; in front of the cluster, along the line of sight to us. If it is any of these the velocity data will be misleading.