The Milky Way has experienced several major mergers over its approximately 13-billion-year history.
Astronomers have discovered a rare event in the universe: three galaxies are gradually coming together and colliding in the system J1218/1219+1035, located approximately 1.2 billion light-years from Earth. The peculiarity of this collision is that all three galaxies have supermassive black holes at their centers, which are currently actively consuming matter.
The results of the study were published in The Astrophysical Journal Letters.
During such consumption, black holes emit powerful radio radiation and launch particle jets known as jets. This means that all three objects are active galactic nuclei (AGN) — a phase when a black hole is intensely "feeding" on surrounding matter.
Astrophysicist Emma Schwartzman from the U.S. Naval Research Laboratory explained the significance of the discovery as follows:
"Triple active galaxies are extremely rare, and observing such a system during the merger gives us a unique opportunity to see how massive galaxies and their black holes grow together."
According to the researchers, the fact that all three black holes are glowing in the radio range translates the existence of triple radioactive nuclei from theoretical models into real observations and opens new opportunities for studying the evolution of supermassive black holes.
Galaxy mergers in the universe occur quite frequently and are considered one of the main mechanisms for their growth. Even the Milky Way has experienced several major mergers over its approximately 13-billion-year history. At the same time, triple mergers are significantly rarer, as all three galaxies must come together simultaneously rather than sequentially.
An even rarer situation is when the nuclei in all three galaxies are active at the same time. The system J1218/1219+1035 has become only the third such finding in the relatively nearby universe and the first where all three AGN are prominently manifested specifically in the radio range.
The system was first noticed in data from the Wide-field Infrared Survey Explorer (WISE) space infrared telescope. Initially, it appeared as a merger of two galaxies that were already partially overlapping. Further observations showed that these two galaxies with active nuclei are separated by about 74,000 light-years. Subsequently, astronomers discovered a third galaxy with an active nucleus at a distance of about 316,000 light-years. The stream of gas extending from it to the other two confirmed that all three objects are participating in a joint merging process.
Researchers note that such systems are important "laboratories" for understanding how galaxies and black holes form and grow. Therefore, observations will continue across various ranges of radiation, not only to study this trio in more detail but also to learn how to find other similar systems. In the scientific article, the authors conclude that these observations confirm the triple nature of the active nuclei in the system and demonstrate the importance of using various methods and ranges of observations to search for such rare objects.
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