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Linné on line arrow Physics and the Cosmos arrow Macrocosmos arrow Galaxies arrow Clusters of Galaxies

Clusters of Galaxies

Galaxies are grouped into larger structures, called clusters or super clusters, in the same way as stars are grouped into galaxies. These clusters do not have the same well defined structure as galaxies but are irregular in their shape. If this is the truth or if it possibly depends on the fact that we have problems to get an overview of so large structures is hard to say.

Are clusters of galaxies the largest structure in the Universe? This is a question that we cannot answer based on what we know today. In order to be able to answer this question we need to improve our measurements and collect much more data. It remains an open question whether this would reveal larger structures.

What is the origin of the clusters of galaxies that we see today? Most theories try to explain this by inhomogeneities, that is irregularities, in the early Universe that have been amplified with time through the influence of gravitation.



The distribution of galaxies out to 150 Mpc according to CfA
 

The figure shows the distribution of galaxies out to 150 Mpc (megaparsec), which roughly corresponds to 2% of the size of the Universe. One can clearly see clusters of galaxies and also areas with low density of galaxies. The two black areas to the left and the right of the centre are not empty areas. Instead they have not been studied because they coincide with the galactic plane of the Milky Way, which disturbs the observations due to the large amount of light from its stars.

The cluster that the Milky Way belongs to is called the local cluster. It consists of approximately 20 galaxies. Other clusters of galaxies that have been studied are the Virgo and Coma clusters. The Coma cluster is one example of a cluster with a regular spherical structure, whereas the Virgo cluster, which is the closest one, does not have same well behaved form.




Photograph of the Coma cluster.

Precisely in the same way as for galaxies, the visible (luminous) matter is too small to be able to keep the cluster together. This means that there must also be dark matter (non luminous). One also has experimental indications for this, based on the phenomenon of gravitational lensing, when studying objects further away in the Universe.