Galaxy Clusters: Busy Neighborhoods in the Universe

The massive galaxy cluster Abell 2744 contains several hundred galaxies clustered together and looking as they did some three-and-a-half billion years ago. NASA/STScI/J. Lotz, M. Mountain, A Koekemoer, HFF team, ESA.  

You've probably heard of galaxy clusters. Just as many stars cluster together, galaxies do, too, although for slightly different reasons. And, when galaxies merge, spectacular things happen, particularly when the gases in and around galaxies combine together to create huge bursts of star birth called "starburst knots".

Our own Milky Way is part of a small collection called the "Local Group", which is itself part of a larger collection called the Virgo Supercluster of galaxies, which is itself part of a large collective of superclusters called Laniakea. The Local Group has at least 54 galaxies, including the nearby spiral Andromeda Galaxy, as well as some smaller dwarf galaxies that appear to be merging with our own galaxy.  

The Virgo Supercluster has around a hundred galaxy groups. Galaxy clusters obviously contain galaxies, but they also harbor clouds of hot gas.  All of the stars and gas that make up galaxy clusters are embedded in "shells" of dark matter -- that unseen material that astronomers are still trying to define. 

Galaxy clusters and superclusters play an important role in helping astronomers understand the evolution of the universe -- from the Big Bang to the present day. In addition, figuring out the origin and evolution of galaxies in clusters, and the clusters themselves may give important clues about the future of the universe. 

Clusters grow as galaxies group together, usually through collisions of smaller clusters. How do they start to form? What happens during their collisions? These are questions that astronomers are answering. 

Probing Galaxy Clusters

The tools of galaxy cluster studies are giant telescopes -- both on Earth and in space. Astronomers focus on light streaming from the galaxy clusters -- many at great distances from us. The light isn't just the optical (visible) light that we detect with our eyes, but also ultraviolet, infrared, x-ray, and radio waves. In other words, they study these distant clusters using nearly the entire electromagnetic spectrum to define the processes going on in these clusters.

For example, astronomers have looked at two galaxy clusters called MACS J0416.1-2403 (MACS J0415 for short) and MACS J0717.5+3745 (MACS J0717 for short) in multiple wavelengths of light. These two clusters around 4.5 to 5 billion light-years from Earth, and it appears that they are colliding. It also appears that MACS J01717 is itself a product of collisions. In a few million or billion years all of these clusters will be one giant cluster.
Astronomers combined all the observations of these clusters into the image seen here, which is of MACS J0717. They come from NASA’s Chandra X-ray Observatory (diffuse emission in blue), Hubble Space Telescope (red, green, and blue), and the NSF’s Jansky Very Large Array (diffuse emission in pink). Where the x-ray and radio emission overlap the image appears purple. Astronomers also used data from the Giant Metrewave Radio Telescope in India in studying the properties of MACS J0416.
The Chandra data reveal super-hot gases in the merging clusters, with temperatures ranging up to millions of degrees.

Visible light observations give us a view of the galaxies themselves as they appear in the clusters. There are also some background galaxies that show up in the visible light images, as well. You might notice that the background galaxies appear somewhat warped-looking. This is due to gravitational lensing, which happens as the gravitational pull of the galaxy cluster and its dark matter "bends" the light from the more distant galaxies. It also magnifies the light from these objects, which gives astronomers another tool to study THOSE objects. Finally, the structures in the radio data trace enormous shock waves and turbulence that is sweeping through the clusters as they merge. Those shocks are similar to sonic booms, generated by the mergers of the clusters.

Galaxy Clusters and the Distant, Early Universe

The study of these merging galaxy clusters is of just one tiny area of the sky. Astronomers actually see such merger activity in nearly every direction of the sky. The idea now is to look farther and deeper in the universe to see earlier and earlier mergers. This requires long observation times as well as more sensitive detectors. As you look farther away in the universe, the harder these become to see because they are so distant and so faint. But, there is amazing science to be done at the earliest frontiers of the cosmos. So, astronomers will keep peering across the depths of space and time, looking for the first mergers of the first galaxies and their infant clusters.