Astronomers have observed a rare system where three supermassive black holes in merging galaxies are combining into one. All three black holes are actively feeding and emitting radio waves, providing a unique glimpse into complex cosmic mergers. This discovery, led by researchers using radio observatories, sheds light on how such massive objects grow.
The discovery involves three galaxies, each harboring a supermassive black hole at its center, that are in the process of colliding to form a single giant galaxy. Such mergers are thought to be key to the growth of supermassive black holes, which can only be detected when they are actively accreting material and emitting light. However, these events are brief compared to the black holes' billion-year lifespans, making them hard to observe. Previously, astronomers had identified only about 150 pairs of merging black holes.
Emma Schwartzman at the US Naval Research Laboratory in Washington DC, along with her team, spotted this triple system. The black holes were detected through low-frequency radio waves that penetrate obscuring dust. Observations came from the Very Long Baseline Array in Hawaii and the Very Large Array in New Mexico, confirming the signals originated from the black holes rather than stellar activity in galaxies.
"The more galaxies involved, the rarer the system gets," Schwartzman noted. She highlighted the novelty: "What’s really interesting is that all three of these [black holes] emit in the radio regime, which we’ve never seen before." The galaxies show early interaction signs, with two separated by 70,000 light years and the third 300,000 light years away.
Isabella Lamperti at the University of Florence in Italy pointed out the early stage of the merger. Emma Kun at Ruhr University Bochum in Germany described it as "like catching the final moments of a galaxy-merging soap opera." Simulating three-black-hole mergers is challenging, but this observation will help physicists understand the dynamics. "This is the first step of finding some physics about the system," Kun added.
The findings appear in Astrophysical Journal Letters (DOI: 10.3847/2041-8213/ae2002).