An international team of astronomers has caught sight of an unusual phenomenon in the merger of two galaxy clusters. Inside this cosmic collision, the researchers saw a galaxy leaving behind a trail of gas, which in itself is not unusual. What is unusual is that the gas is not dispersing, and that the further it is from the galaxy, the brighter it appears.
The finding, reported in Science Advances, has puzzled scientists. A bit like an airplane trail, the gas should disperse and spread out in the intergalactic space. Instead, the trail has remained in place for a very long time, with an unknown process giving it energy.
"This was totally unexpected," lead author Francesco de Gasperin, from Leiden University, said in a statement. "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."
He added: "Part of the energy released in the merger event must have been transferred to rejuvenate the cloud of electrons."
The brightest part of the trail is located towards the central region of the cluster. The cluster itself is known as Abell 1033, located 1.6 billion light-years from Earth. Abell 1033 already had its 15 minutes of fame when, a few years back, it was nicknamed a “radio phoenix” – an inactive cluster suddenly reborn in radio wave thanks to its merger with another group of galaxies.
Radio waves were also crucial for this study. The team used the Low-Frequency Array (LOFAR) and were able to probe frequencies that had not been observed before.
"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," explained 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."
It will be interesting to know if this mechanism is the exception or the norm in intergalactic plasma. More low-frequencies studies might reveal just that.