Rare Triple Galaxy Merger J1218/1219+1035 Reveals Three Radio-Bright AGN

Around 1.2 billion light-years from Earth, an enormous event is taking place on a cosmic scale.

J1218/1219+1035: a rare triple galaxy merger

Three galaxies, rather than the usual two, are slowly converging in a vast triple collision known as J1218/1219+1035. Such an arrangement is already unusual – yet this event has an additional remarkable feature.

At the centre of each galaxy, a supermassive black hole is actively consuming matter and emitting bright radio light in the process.

Astrophysicist Emma Schwartzman of the US Naval Research Laboratory says: "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."

"By observing that all three black holes in this system are radio-bright and actively launching jets, we've moved triple radio active galactic nuclei (AGN) from theory into reality and opened a new window into the life cycle of supermassive black holes."

Galaxy mergers occur relatively often across the Universe and are considered one of the principal ways in which galaxies and their resident supermassive black holes increase in size. Over its 13 billion-year history, the Milky Way itself appears to have undergone at least three or four major mergers.

Astronomers have recorded many mergers involving pairs of galaxies in the nearby Universe, whereas three-galaxy systems are much less common. They depend on all three galaxies merging simultaneously, rather than through a staggered, hierarchical process.

Three radio-bright active galactic nuclei

Finding AGN in all three galaxies is even less likely. J1218/1219+1035 is only the third such system identified in the nearby Universe, and it is the first where each of the three AGNs emits radio light.

The system emerged from data gathered by the Wide-field Infrared Survey Explorer, which marked it as unusual. At first, it appeared to be a merger of two galaxies whose edges were already overlapping, with an AGN in each – a noteworthy discovery in itself.

Subsequent observations verified that the two adjoining galaxies do host AGN, positioned roughly 74,000 light-years apart. The team also unexpectedly identified a third galaxy containing a luminous AGN, about 316,000 light-years away. A stream of gas seemingly travelling from this third galaxy towards the other pair confirms that it belongs to this rare triple system.

Because triple galaxy mergers offer valuable laboratories for studying the growth of galaxies and black holes, astronomers will conduct follow-up observations at additional wavelengths. Their aim is not only to investigate this particular triplet in greater detail, but also to establish how other systems of this kind might be found while still hidden in plain sight, awaiting the observations that reveal them.

The researchers write: "These observations 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."

The results were published in The Astrophysical Journal Letters.