Astronomers may have uncovered a hidden supermassive black hole inside the famous Antennae galaxies NGC 4038/4039, a pair of colliding galaxies best known for their spectacular bursts of star formation. The paper outlining the findings was posted to the arXiv preprint server on May 21.
A popular target
The Antennae galaxies are a pair of colliding spirals located roughly 70 million light-years from Earth, making them the nearest example of two gas-rich galaxies merging. The system is also one of the youngest of its kind. The two merging galaxies are called Antennae galaxies because the collision has stripped out long tails of stars, gas, and dust from them, resembling an insect’s antennae. The interaction has triggered one of the most intense bursts of star formation seen in the local universe, making the Antennae a widely used laboratory for studying how galaxies grow through interactions.
Galaxy mergers are known to do more than just ignite star formation. The gravitational disruption they cause can funnel gas toward galactic centers and trigger the supermassive black holes residing there to accrete, turning into an active galactic nucleus (AGN). The Antennae, however, only showed a spectrum dominated by starburst activity so far, with no indication that either black hole was actively feeding. A previous study analyzing the massive star clusters in the merging system, however, found some changes in the brightness near the core of one of the galaxies. Astronomers are now curious to know if this has anything to do with the AGN activity.
Looking for flickers
In this new study, a team of astronomers led by Shinya Komugi of Kogakuin University, Japan, used ALMA to observe Antennae galaxies at 100 GHz. They observed the system 52 separate times over about 2.5 months. By doing this, the team specifically searched for time variability—changes in flux between observations.
Rapid variability in millimeter emission can say a lot about the physical size of the emitting region. Because information cannot travel faster than light, changes in brightness cannot spread instantly across a large object. So, for example, if astronomers see a source change brightness in just 2 days, the region producing that light must be smaller than the distance light can travel in 2 days—about 2 light-days across.
The observations shed light on two compact sources near the core of NGC 4039, designated S3 and S4. S3 appears slightly extended and has a luminosity exceeding that of typical supernova remnants or X-ray binaries. However, it shows no variability. So, the team thinks it could be originating from a kind of radiation from a young massive star cluster, though an AGN origin is not ruled out.
Shrouded AGN
S4 is more striking. It exhibits a variation in its flux on a timescale of just 13 days. This rate of variability limits the emitting region’s size to less than 13 light-days or 0.01 parsecs across. This region is too compact for a star-forming region, supernova remnant, or dust cloud. Researchers explain that the limit on the size corresponds to roughly a Schwarzschild radius matching a supermassive black hole with a mass of 10 million suns.
Additionally, its brightness temperature at the observing frequency exceeds one million Kelvin. If the light was produced by thermal processes, such as those associated with star formation, the temperature would not be expected to reach such high values. Instead, the high temperature points toward a non-thermal origin, likely linked to energetic activity near a supermassive black hole.
If any of these objects were actually an AGN, they should emit “hard” high-energy X-rays. However, neither S3 nor S4 are detected in hard X-rays. The researchers suggest that this is only possible if the AGN is a Compton-thick AGN: a core so deeply buried in gas and dust that even high-energy X-rays cannot escape.
“Our observations could indicate that there could be an obscured AGN already in an early-phase interacting galaxy,” they write in the new paper.
The researchers caution that confirmation will require more observations in a wider frequency range to better understand the emissions of both sources. Follow-up infrared spectroscopy with JWST and X-ray observations with NuSTAR may ultimately reveal whether S4 is truly the hidden black hole the variability data suggests.
For now, the study provides an updated picture of the Antennae system. Rather than a pure starburst, the merger may host a deeply obscured, actively feeding supermassive black hole already at this relatively early stage of interaction.
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Publication details
Shinya Komugi et al, An AGN in the Antennae galaxies ?, arXiv (2026). DOI: 10.48550/arxiv.2605.21879
Journal information:
arXiv
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A hidden supermassive black hole may be lurking inside the Antennae galaxies (2026, May 29)
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