Clusters Background Information: Galaxy Clusters | ![]() |
Galaxies are not merely strewn randomly throughout space. Instead, they often come in groups or clusters ranging from small associations of a few large galaxies to giant clusters which contain thousands of galaxies. Our own Milky Way galaxy is a member of a small group called the Local Group, consisting of the Milky Way, the two other large spirals M31 (the Andromeda galaxies) and M33 (the Pinwheel), and a number of small satellite galaxies.
The nearest large cluster of galaxies is the Virgo Cluster, approximately 18 million parsecs (Mpc) away.1 At the center of the Virgo Cluster is the giant elliptical galaxy M87. Even further away is the dense, massive Coma Cluster (100 Mpc distant).
Galaxies within clusters are bound together by their mutual gravitational pull. Unlike the rest of the Universe, which expands with the cosmic expansion since the Big Bang, galaxies in clusters do not expand away from each other, but rather orbit around the cluster center on timescales of billions of years. As they orbit inside the cluster, galaxies often pass by each other enough to strongly distort one another by their gravitational pull. In extreme cases, galaxies can actually capture each other and merge together, in a process which transforms spiral galaxies into ellipticals. As these interactions between cluster galaxies occur, the galaxy population is slowly changed from the spiral-dominated galaxy population (observed outside of clusters) into the elliptical dominated populations found in rich clusters. Clusters of galaxies are interesting laboratories for studying galaxy evolution, and can also tell us much about the evolution of the Universe in general. The rate at which clusters form and evolve depends on the density of matter in the Universe -- if the density is low, there is not enough mass to keep clusters forming continuously throughout cosmic history. By studying clusters we can learn about how the Universe itself evolves. Clusters also tell us about the mysterious dark matter which permeates the Universe. By studying the velocities of galaxies in clusters, we can measure the amount of mass contained inside the cluster. Comparing this "dynamical mass" to the amount of mass we infer just by counting galaxies, we find that much of the mass in clusters is not contained within the galaxies themselves. Even when we account for the hot intercluster gas (which emits X-rays, not visible light), we still come up with only 10% of the total mass we infer from the galaxy velocities. The "missing mass", comprising 90% of the mass in galaxy clusters, is the mysterious dark matter which we still do not understand. In this JavaLab, we will use the velocities and spatial distribution of galaxies in clusters to get an estimate of the dynamical mass of clusters. |