Galaxy clusters are monstrous things, but they can be difficult to detect as they get further and further away. However, the interesting physics that occurs in these clusters has made a new galaxy cluster visible to two orbiting space telescopes, Planck and XMM-Newton.

Galaxy clusters appear to be exactly what they sound like: groupings of galaxies physically bound together by gravity. There is, however, so much more that they eye cannot see. Galaxy clusters appear to be dominated by dark matter, which was famously detected in the merging galaxy clusters known collectively as the Bullet Cluster. But neither Planck nor XMM-Newton are set up to detect that.

If you just look at the "normal" matter in a cluster, the galaxies themselves only make up a small percentage. The cluster is instead dominated by hot gas between the galaxies. In fact, the mass of this gas is typically around five times the mass of all the galaxies in the cluster put together! This gas is so hot that it glows in X-rays, which is why many distant clusters have been detected by x-ray telescopes.

Planck, however, works in the radio and microwave regions of the electromagnetic spectrum. Its mission is to study the cosmic microwave background (CMB), or the signal from when the universe was just 300,000 years old. In order to do that, astronomers need to weed out any sources that may lie in front of the CMB, such as emission from our own Milky Way Galaxy. Galaxy clusters also show up in the microwave in their own way.

Photons (or, roughly speaking, particles of light) from the CMB travel across vast distances and may slam into a galaxy cluster. They specifically can hit particles of the hot gas in the cluster. Hot gas moves quickly, and some of that energy is imparted to the photon in a process called "scattering." The photon then flies off to continue its journey, now with higher energy and higher frequency. In this way, when you look at the CMB in lower frequencies, galaxy clusters show up as dark spots or "holes," whereas at higher microwave frequencies, they are bright spots. This process is called the Sunyaev-Zel'dovic Effect, or SZE.

Planck is in the process of detecting galaxy clusters with the SZE as it has frequency coverage both there they show up as holes and bright spots. The scientists then go and match these spots to known clusters. One such cluster seen with the SZE had no known counterpart, so astronomers pointed the x-ray telescope XMM-Newton and did detect the cluster (see top image). In fact, it's a real whopper, being called a supercluster as it appears to be comprised of three smaller galaxy clusters.