Galaxy mergers are among the most spectacular events in the Universe. During these encounters, not only billions of stars are redistributed, but the supermassive black holes lurking in the galaxies' centres can also interact with one another. Systems in which three galaxies are merging simultaneously, however, are quite rare. An international team of Chinese and Hungarian researchers, led by doctoral student Wancheng Xu from the Xinjiang Astronomical Observatory of the Chinese Academy of Sciences – currently spending a one-year research fellowship at the Konkoly Observatory of the HUN-REN Research Centre for Astronomy and Earth Sciences (CSFK) – has now investigated one such remarkable system, the galaxy group UGC 2369S. Their study provides direct evidence that one of its galactic nuclei harbours an active supermassive black hole accreting matter from its surroundings.
Located at a relatively modest cosmological distance of about 450 million light-years, UGC 2369S is rich in interstellar gas and dust. As a result, observations at optical wavelengths are severely hindered because thick dust clouds obscure the central regions of the system. To overcome this limitation, the researchers turned to radio astronomy, employing the technique of Very Long Baseline Interferometry (VLBI), which combines observations from radio telescopes separated by up to thousands of kilometres across the Earth to achieve extremely high angular resolution. By analysing archival data from the European VLBI Network (EVN) and the U.S. Very Long Baseline Array (VLBA), they mapped the galactic nuclei in the system with milliarcsecond resolution.
Like the archives of many astronomical observatories, the databases of these radio telescope arrays contain a wealth of valuable observations that were never fully analysed or published by the original observing teams. In this case, the researchers revisited observations obtained at two different radio frequencies during three observing sessions in 1996 and 2003. These archival data revealed a compact radio source whose properties clearly indicate that the detected emission originates not from extended star formation but from an active galactic nucleus (AGN).
Fig. 1. The merging triple-galaxy system UGC 2369S as observed in the near infrared with the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope. The overlaid contours show the X-ray intensity measured by the Chandra X-ray Observatory in the 0.5–7 keV energy range. The scale bar in the lower left corresponds to approximately 3,300 light-years. The compact radio source detected with VLBI is located within the northern galactic nucleus, which is also the brightest in X-rays. (Credit: Y. Ding et al., 2026)
Perhaps the most intriguing result is that this AGN, despite its relatively modest power output, launches relativistic plasma jets. The 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 data 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.
Fig. 2. The highest-resolution VLBI image of the northern nucleus of UGC 2369S, obtained with the VLBA at 5 GHz. The white bar in the lower right corresponds to 10 milliarcseconds, equivalent to only 21.5 light-years at the distance of the galaxy. In other words, this image zooms in by roughly a factor of 150 compared with the previous figure. (Credit: W. Xu et al., 2026)
These 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.
The results have been accepted for publication in the journal Astronomy & Astrophysics:
Xu W., Frey S., Cui L., Gabányi K.É. (2026): Direct VLBI Evidence for a Buried Active Galactic Nucleus in the Triple-Merger Luminous Infrared Galaxy UGC 2369S. Astronomy & Astrophysics, https://doi.org/10.1051/0004-6361/202660763