J1218/1219+1035 Reveals a Rare Triple Active Galaxy Merger

Around 1.2 billion light-years from Earth, an enormous event is playing out across the cosmos.

J1218/1219+1035: a rare triple galaxy merger

At this location, three galaxies – rather than two – are slowly converging in a vast triple collision designated J1218/1219+1035. Such an encounter is exceptionally unusual in itself, yet this one has an additional remarkable feature.

The supermassive black hole at the centre of each galaxy is actively consuming matter. As they feed, all three are emitting brightly at radio wavelengths.

"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 radio active galactic nuclei (AGN) from theory into reality and opened a new window into the life cycle of supermassive black holes."

Why triple active galaxies matter

Galaxy mergers occur fairly often across the Universe and are considered one of the principal ways in which galaxies – along with the supermassive black holes they contain – increase in size. The Milky Way, for example, bears signs of at least three or four major mergers during its 13-billion-year history.

Astronomers have identified many mergers involving pairs of galaxies in the nearby Universe. Triples, however, are much less common, because all three galaxies must merge simultaneously instead of through a staggered, hierarchical process.

Rarer again is a system in which each of the three galaxies contains an AGN. J1218/1219+1035 is only the third such example found in the nearby Universe, as well as the first where every one of its three AGNs is radio-bright.

Observations of the three radio-bright AGNs

The unusual system emerged in data gathered by the Wide-field Infrared Survey Explorer. At first, it appeared to be a two-galaxy merger: the galaxies were already overlapping at their outer edges, and each seemed to host an AGN – a compelling discovery in its own right.

Subsequent observations verified that the two intersecting galaxies do contain AGNs, with a separation of roughly 74,000 light-years. The researchers then unexpectedly detected a third participating galaxy, also home to a luminous AGN, around 316,000 light-years away. A stream of gas apparently moving from this third galaxy towards the other pair supports its membership of this rare triple system.

Because triple galaxy mergers provide excellent natural laboratories for investigating the growth of galaxies and black holes, astronomers intend to examine the system at additional wavelengths. The aim is both to learn more about this individual triplet and to establish better ways to identify comparable systems that may be in plain sight, awaiting the observations that reveal them.

"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."

The findings were published in The Astrophysical Journal Letters.