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10:45:15 
2025-12-31 
1597
Some 1.2 billion light-years from Earth, a massive event is unfolding on cosmic scales.
There, not two, but three galaxies are gradually coming together in a giant, triple collision named J1218/1219+1035. That's rare enough on its own – but this particular event is even more special.
Each of the three supermassive black holes lurking at the cores of their respective galaxies is actively guzzling down material, blazing with radio light as it does so.
"Triple active galaxies like this are incredibly rare, and catching one in the middle of a merger gives us a front-row seat to how massive galaxies and their black holes grow together," says astrophysicist Emma Schwartzman of the US Naval Research Laboratory.
"By observing that all three black holes in this system are radio-bright and actively launching jets, we've moved triple radioactive galactic nuclei (AGN) from theory into reality and opened a new window into the life cycle of supermassive black holes."
Galaxy mergers are not uncommon throughout the Universe; in fact, they're thought to be one of the major mechanisms whereby galaxies, and their incumbent supermassive black holes, grow. The Milky Way itself shows evidence of at least three or four major mergers over its 13 billion-year lifespan5.
Astronomers have cataloged a significant number of mergers between pairs of galaxies in the nearby Universe, but galaxy triples are a lot rarer. They require all three galaxies to merge at the same time, rather than a staggered, hierarchical event.
For all three galaxies to have AGN is rarer still; J1218/1219+1035 is just the third ever discovered in the nearby Universe, and the first in which all three AGNs shine in radio light.
The system was spotted in data collected by the Wide-field Infrared Survey Explorer, and flagged as unusual. Initially, it looked like a merger between two galaxies, already overlapping at the edges, each with its own AGN – interesting enough in its own right.
Follow-up observations confirmed that these two touching galaxies are indeed host to AGN, separated by a distance of around 74,000 light-years. Surprisingly, however, the researchers found a third galaxy with a blazing AGN also involved, located about 316,000 light-years away. A tail of gas that appears to be flowing from this galaxy to the other two confirms its place in this rare triple.
Since triple galaxy mergers are excellent laboratories for understanding how galaxies and black holes grow, astronomers are going to follow up in more wavelengths, not just to learn more about this particular triplet, but to understand how to look for other such systems that may be hiding in plain sight, waiting for the right observation to catch them in the act.
"These observations," the researchers write, "confirm the triple AGN nature of this system and highlight the necessity for diverse and multiwavelength selection strategies in the continued search for these rare systems."
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