Galaxy mergers are among the most spectacular and destructive events in the universe. During these encounters, not only are billions of stars redistributed, but the supermassive black holes lurking in the galaxies' centers can also interact with one another. Yet systems where three galaxies merge simultaneously are uncommon.

Now, researchers have obtained the first direct observational evidence for an active galactic nucleus (AGN) in the triple-merger galaxy UGC 2369S. Deep within the northern nucleus of this complex system lurks a supermassive black hole steadily accreting matter from its surroundings.

The observations were led by XU Wancheng, a PhD student at the Xinjiang Astronomical Observatory (XAO) of the Chinese Academy of Sciences (CAS) — currently a visiting researcher at the Konkoly Observatory in Hungary — under the supervision of Dr. Sándor Frey from Konkoly Observatory and Prof. CUI Lang from XAO.

The relevant findings have been published in the journal Astronomy & Astrophysics.

UGC 2369S is a triple-merger luminous infrared galaxy located about 450 million light-years from Earth. The system is rich in interstellar gas and dust, and these thick clouds heavily obscure the central regions, effectively "blinding" conventional optical observations.

To get around this, the researchers turned to radio astronomy and very long baseline interferometry (VLBI) — a technique that combines data from radio telescopes separated by up to thousands of kilometers to achieve extremely high angular resolution. Using archival data from the European VLBI Network (EVN) and the US Very Long Baseline Array (VLBA), they mapped the galactic nuclei at milliarcsecond resolution.

By analyzing the L- and C-band VLBI datasets obtained during 1996 and 2003, which were never fully analyzed, the researchers identified a compact radio source associated with the northern nucleus of UGC 2369S. Its unique physical characteristics showed that the radio emission was produced by an AGN rather than by extended star formation.

According to the researchers, the most intriguing result is that this AGN, despite its relatively modest power output, launches relativistic plasma jets. Its central black hole is enshrouded by a very dense cocoon of gas and dust, rendering it almost invisible at optical wavelengths. Radio waves, however, penetrate this obscuring material, allowing the VLBI observations to identify the hidden nucleus directly.

The results also suggest that the black hole is currently accreting matter at a relatively low rate while simultaneously injecting a substantial amount of energy back into its galactic environment through its radio-emitting jets.

The researchers said the findings have implications that extend well beyond this single system. Galaxy mergers are a fundamental driver of cosmic evolution, shaping both the growth of supermassive black holes and the history of star formation in galaxies. UGC 2369S demonstrates that even the most heavily obscured galactic nuclei can host active black holes that remain virtually undetectable by means of conventional optical observations. VLBI provides one of the most powerful tools available for uncovering these hidden, jet-producing black holes.