Back in 1971, a couple of British astronomers predicted the existence of a black hole at the center of our galaxy. And in 1974, other astronomers found it, naming it Sagittarius A*.
Since then, astronomers have discovered that a similar “supermassive black hole” sits at the center of almost every other large galaxy. In 2019, they took the first image of a supermassive black hole. Today, these exotic objects are a fundamental part of our understanding of how galaxies form and evolve.
But what of smaller astronomical bodies, like the Large Magellanic Cloud, a dwarf satellite galaxy that is expected to collide with the Milky Way in 2.4 billion years? Nobody is quite sure whether clouds like this might also house supermassive black holes.
Galactic Evolution
Now the evidence is beginning to stack up, thanks to the work of Jiwon Jesse Han at the Harvard-Smithsonian Center for Astrophysics in Cambridge and colleagues, who have uncovered compelling evidence that a supermassive black hole resides in the Large Magellanic Cloud after all. If confirmed, the discovery would challenge conventional astrophysical models and deepen our understanding of galaxy evolution.
The key to their discovery comes from the study of hypervelocity stars moving so quickly that they can escape the gravitational pull of the Milky Way.
Astrophysicists believe these stars began life as one half of a binary system that strayed too close to Sagittarius A*. The extreme gravitational forces in this interaction sent one star hurtling into space while the other remained bound to the black hole.
Astronomers have spotted 21 hypervelocity stars in the last two decades, all of them B-type main-sequence stars that are more massive, luminous and bluer than the Sun (these stars are easier to spot than other stars in these kinds of surveys).
More recently, they have also been able to determine the proper motion of these stars. So Han and co decided to investigate their origin by rewinding their proper motion to see where they came from.
That led to an unexpected discovery: many of these stars do not trace back to Sagittarius A* at all. Instead, their trajectories suggest they came from the Large Magellanic Cloud. “We find that half of the unbound hypervelocity stars discovered by the HVS Survey trace back not the Galactic Center, but to the Large Magellanic Cloud,” say Han and co.
In particular, Han and co found a cluster of these stars in the direction of the constellation Leo—the so-called "Leo Overdensity”.
This kind of clustering is not easy to explain if the stars came from the Milky Way. One possibility is that they formed when one of a binary pair became a supernova, accelerating the other to huge velocities; another is that these stars are the result of some fantastical slingshot effect that occurs when three or four stars come together at the same time.
But none of these mechanisms can produce stars with such high velocities in such a concentration, say Han and co. Instead, the most plausible explanation is that they were launched by a supermassive black hole at the center of the Large Magellanic Cloud. “We find that the birth rate and clustering of Large Magellanic Cloud hypervelocity stars cannot be explained by supernova runaways or dynamical ejection scenarios not involving a supermassive black hole,” say the team.
Pattern Matching
To test the hypothesis, Han and co simulated the way a supermassive black hole in the Cloud would interact with nearby star systems. It turns out that it would spit out stars in a pattern that closely matches the observed data, in particular, producing the Leo Overdensity. “The predicted spatial and kinematic distributions of simulated hypervelocity stars are remarkably similar to the observed distributions,” say Han and co.
The team estimate that the mass of the Large Magellanic Cloud’s putative black hole to be around 600,000 solar masses—significantly smaller than Sagittarius A*, which is about 4.3 million solar masses, but still within the range of known SMBHs.
If confirmed, this would make the Large Magellanic Cloud one of the smallest galaxies known to host a supermassive black hole. It suggests the formation of supermassive black holes must be more common than expected and could also explain other long-standing anomalies in the dynamics of the Cloud.
For example, astronomers have observed unusual motions of stars and unexplained mass distributions within it, which could be the result of the gravitational pull of a central black hole. In other words, this supermassive black hole could have played a crucial role in shaping the Cloud’s internal structure and its interaction with the Milky Way.
Of course, more evidence will be needed to confirm the discovery. Future observations with high-resolution telescopes or next-generation space-based observatories could help detect the signature emissions from a black hole or its gravitational influence on nearby stars.
And the discovery of more hypervelocity stars, particularly in the southern hemisphere, could strengthen the argument. If more of these trace back to the Cloud, it would provide further confirmation that a black hole is at work.
In the much longer term, the Milky Way is destined to have a closer relationship to the Large Magellanic Cloud and its black hole—their current velocities suggest they will collide in about 2.4 billion years. In the meantime, the search for this black hole is set to begin in earnest.
Ref: Hypervelocity Stars Trace a Supermassive Black Hole in the Large Magellanic Cloud : arxiv.org/abs/2502.00102
