Clusters of Galaxies
Most galaxies are not alone in the vast expanse of space, but are connected to one or more other galaxies by gravity. The same force that holds you onto the Earth can keep many individual galaxies bound together. Groups can be small, such as two galaxies orbiting each other, or large, like the rich Coma cluster of thousands of galaxies extending for more than ten million light years. These are the largest objects in the known Universe, and they have many properties that make them great astrophysical laboratories. For example,
- clusters change very slowly (it takes almost as long as the age of the Universe for significant changes to occur in clusters), thus clusters retain an imprint of how they were formed. This makes them a good probe of the history of structure and galaxy formation.
- clusters tend to hold onto the gas in their systems, unlike galaxies, where the gas is forced out through supernova explosions. In other words, clusters are closed systems. By studying the chemical composition of clusters, it is possible to get a history of nucleosynthesis in the Universe.
- the force of gravity that holds clusters together comes mostly from dark matter, making clusters an excellent way to study dark matter in the Universe.
The most visible part of galaxy clusters, all of the stars in all of the galaxies that make up the cluster, is a small fraction of the sum total of what makes up the cluster, and is probably the least interesting part of the cluster. For example, scientists study the X-ray emission from galaxy clusters. The X-rays come from hot (10-100 million degrees) gas trapped by the gravitational force of the cluster. This gas makes up a much larger part of the total mass of the cluster than the stars, but is completely invisible to human eyes!
The scientists at Goddard Space Flight Center are studying many different things about galaxy clusters, but these can be divided into two main groups: