An international team of astronomers led by Francesco de Gasperin (Hamburg and Leiden University) has witnessed an unexpected phenomenon in a merger of two clusters of galaxies. The astronomers discovered a gas trail that slowly extinguished but then lit up again. It is unclear where the energy for the rejuvenation of this trail comes from. 

The astronomers investigated Abell 1033. This is a cluster of galaxies consisting of two smaller clusters that are in the process of merging. Abell 1033 is located in the northern constellation of Leo Minor (near Ursa Major). Clusters of galaxies are the largest structures in the Universe. They can contain hundreds to thousands of galaxies similar to our Milky Way. Smaller clusters can merge together to form a larger cluster.

The astronomers observed that an individual galaxy in Abell 1033 leaves a trail of gas as it traveled through the cluster. On astronomical scale, such a trail resembles the trail of colored smoke behind a stunt plane. It was expected that the gas trail, like the ones behind a stunt plane, would slowly fade and eventually disappear. To their astonishment they saw that the end of the gas trail was brighter than the middle.

"This was totally unexpected," says de Gasperin. "As these clouds of electrons radiate away their energy over time, they should become fainter and disappear. Instead, in this case, after more than a hundred million years, the trail of electrons is glowing brightly."

There is no precise explanation for this phenomenon yet. It seems that the trail brightens near the center of the cluster of galaxies. De Gasperin: "Part of the energy released in the merger event must have transferred to rejuvenate the cloud of electrons.”

This work has been published in Science Advances http://advances.sciencemag.org/content/3/10/e1701634.