An international team of astronomers led by Francesco de Gasperin (Leiden University, the Netherlands) has witnessed an unexpected phenomenon in a merger of a 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 researchers publish their findings in Science Advances.
Published by the editorial team, 4 October 2017
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.
The astronomers had 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 Francesco de Gasperin, the first author of the research paper that is published in Science Advances. "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 been transferred to rejuvenate the cloud of electrons."
The research on merging clusters of galaxies is complicated because astronomers only see a snapshot of a process that takes billions of years to complete. In addition to that, the telescopes that are needed for the investigation must receive signals at extremely low frequencies.
The astronomers combined data from the Indian Giant Metrewave Radio Telescope and LOFAR, the Low Frequency Array. LOFAR was designed and built by the Dutch research institute ASTRON. The telescope consists of thousands of antennas spread across eight countries. The heart of LOFAR is in Drenthe in the north-east of the Netherlands.
"It’s like being among the last explorers. As soon as we move in uncharted territories, or in this case at unexplored frequencies, our universe is still full of surprises," says De Gasperin. "And this is just a first step. Much is still to be done to understand the complexity of galaxy clusters, and find what is lurking at low radio frequencies"
The image shows a galaxy in orange that moves to the left and leaves a gas trail. The trail seems to extinguish slowly, but lightens up again near the second, white-yellow galaxy. Most white dots in the image are complete galaxies. (c) Francesco de Gasperin (Leiden University).
Gentle re-energisation of electrons in merging galaxy clusters. By: F. de Gasperin, H.T. Intema, T.W. Shimwell, G. Brunetti, M. Brüggen, T.A. Enßlin, R.J. van Weeren, A. Bonafede, H.J.A. Röttgering. Accepted for publication in Science Advances. (open access)