Galaxy Cluster

As the Universe cooled after the Big Bang, slight concentrations of matter began to grow into galaxies. The largest concentrations had the most gravitational pull, and were thus able to collect more matter, and grow to enormous proportions. The result were clusters of galaxies, like the one in the top image to the right --- a collection of galaxies that contain in total hundreds of times stars than our own Milky Way.

Galaxy clusters are important for several reasons. They are the largest structures in the Universe, and so they provide insight into how matter collected to form galaxies. They contain a large density of galaxies, and so collisions between galaxies are relatively common. Therefore, one can compare the shapes of galaxies in clusters to galaxies that are more isolated, in order to understand how collisions influence the evolution of galaxies.

However, one of the most important aspects of galaxy clusters is that, because of their enormous mass, they provide insight into the nature of dark matter. It was, in fact, because of the observations of galaxy clusters that the existence of dark matter was first hypothesized. In 1933, Fritz Zwicky (one of 20th century's most important and least-well known astronomers) measured the motions of galaxies in the Coma Cluster, and calculated the amount of mass that would hold that collection of galaxies together. He then added up the light from the galaxies, assumed that the light was produced by stars like our sun, and determined the mass in the galaxies. To his surprise, the galaxies only seemed to contain a few percent of the mass needed to hold the galaxy cluster together. He hypothesized that there must be some "dark matter" holding the galaxies together.

Zwicky was largely ignored, however, because although he was brilliant, he was also obnoxious, and not very well-liked. It took 40 years for other astronomers to find other evidence for dark matter. Several groups began studying the motions of stars within individual, nearby galaxies, and realized that the stars were moving faster than one would expect if only the mass from the stars were holding the galaxies together. With this new discovery, Zwicky's pioneering work on galaxy clusters was revived.

In addition to dark matter, however, galaxy clusters contain more ordinary matter that cannot be seen with an optical telescope like Hubble. The enormous gravity of the cluster can hold matter within its grasp even if it has temperatures up to tens of millions of degrees. At this temperature, matter would be in the form of X-ray emitting plasma. Indeed, telescopes like the Chandra X-ray Observatory reveal the galaxy clusters are bright sources of X-rays. Remarkably, careful study of these X-ray images reveal that there is much more ordinary matter in the hot plasma of galaxy clusters than there is in the galaxies that give them their name.