Finding Out What Clusters Are Made Of
Clusters are made up of two basic types of matter: luminous matter (like stars and hot gas) and dark matter. Dark matter does not shine on its own, and the only way we know it exists is because of its gravitational affect on luminous matter. If we want to know how much dark matter there is in the entire Universe, we have to study something that is representative of the Universe as a whole. Something big, that is. Clusters of galaxies are the largest known objects in the Universe, believed to be big enough that they have the same fraction of dark matter as the whole Universe. One area of cluster research is aimed at using X-ray observations to understand how much luminous matter and dark matter exists in clusters.
Most of the luminous matter in clusters is in the form of hot gas in between the galaxies. The gas, which has a temperature of 10-100 million degrees, radiates X-rays. How much hot gas is in a cluster is simply related to the total X-ray luminosity we observe from the cluster. Thus, we can make a direct measurement of the luminous matter from X-ray observations of clusters of galaxies.
How much dark matter is in a cluster, however, has to be inferred from the observable, luminous matter. We can do this because galaxy clusters are "relaxed" systems, that is, there is a balance between the dark matter and the pressure of the cluster. The pressure of the cluster is related to the X-ray emitting gas (which we observe), so, by assuming an equilibrium between the two, we can estimate the amount of dark matter.
Some Surprising Conclusions
When we study the of the amount of dark matter in clusters, we find that although there is more dark matter than luminous matter in clusters, there is much less dark matter than many "theories of everything" predict. One of the implications of this, if it is true, is that the Universe would keep on expanding forever: there would not be enough matter to halt the expansion with gravity.
Chemical Abundances in Clusters: Other Evidence
Another avenue of current research is to use the X-ray spectra to determine what kinds of elements the gas in between the galaxies contains. These observations can then be used as a further test of models of clusters and their evolution.
The luminous matter in a cluster is not all the same. Some of it is primordial hydrogen and helium created in the Big Bang; some of it is heavier elements like oxygen, neon, magnesium, silicon, and sulfur. These latter elements were created by stars in fusion processes, or in supernova explosions. Models that can explain how these elements got out of the stars inside the galaxies in the clusters and into the intracluster gas might also explain why some clusters have relatively more dark matter than others.
Thank you to Michael Loewenstein for contributing to this article.