Researchers from the Center for Astrophysics, Harvard & Smithsonian have found compelling evidence of the nearest supermassive black hole beyond the Milky Way. This massive black hole resides in the Large Magellanic Cloud, one of the closest neighboring galaxies.
Artist’s impression of a hypervelocity star ejected from the Large Magellanic Cloud (shown on right). When a binary star system ventures too close to a supermassive black hole, the intense gravitational forces tear the pair apart. One star is captured into a tight orbit around the black hole, while the other is flung outward at extreme velocities, often exceeding millions of miles per hour, becoming a hypervelocity star. The inset illustration depicts this process: the original binary’s orbital path is shown as interwoven lines, with one star being captured by the black hole (near the center of the inset) while the other is ejected into space (lower right). Image Credit: CfA/Melissa Weiss
Astronomers tracked the trajectories of 21 stars on the Milky Way's periphery with incredibly fine precision to make this discovery. These stars are moving so fast that they can escape the gravitational pull of the Milky Way and any nearby galaxy. These are known to astronomers as “hypervelocity” stars.
In the same way that forensic specialists reconstruct the origin of a bullet by its trajectory, scientists identified the origin of these hypervelocity stars and discovered that roughly half of them are associated with the supermassive black hole at the Milky Way's center, while the other half came from a previously unidentified giant black hole in the LMC.
It is astounding to realize that we have another supermassive black hole just down the block, cosmically speaking. Black holes are so stealthy that this one has been practically under our noses this whole time.
Jesse Han, Study Lead, Center for Astrophysics, Harvard & Smithsonian
Using information from the European Space Agency's Gaia mission, a satellite that has tracked over a billion stars across the Milky Way with previously unheard-of accuracy, the researchers discovered this elusive black hole. They also made use of new information that other researchers had recently acquired about the LMC's orbit around the Milky Way.
We knew that these hypervelocity stars had existed for a while, but Gaia has given us the data we need to figure out where they actually come from. By combining these data with our new theoretical models for how these stars travel, we made this remarkable discovery.
Kareem El-Badry, Study Co-Author, California Institute of Technology
When a double-star system approaches a supermassive black hole too closely, hypervelocity stars are produced. One star is drawn into a tight orbit around the black hole as the two are torn apart by its powerful gravitational pull. A hypervelocity star is created when the other orphaned star is jettisoned away at speeds greater than several million miles per hour.
One important aspect of the team's study was their theoretical model's prediction that due to the way the LMC orbits the Milky Way, a supermassive black hole in the LMC would produce a cluster of hypervelocity stars in one area of the Milky Way. The stars that are expelled in the direction of the LMC's motion ought to gain additional velocity. Their data showed that such a cluster existed.
The team found that other mechanisms—such as stars being ejected when their companions undergo a supernova or through a process similar to that in double star systems but without a supermassive black hole—are insufficient to explain the properties of hypervelocity stars.
The only explanation we can come up with for this data is the existence of a monster black hole in our galaxy next door. So in our cosmic neighborhood it is not just the Milky Way’s supermassive black hole evicting stars from its galaxy.
Scott Lucchini, Study Co-Author, Center for Astrophysics, Harvard & Smithsonian
Based on the relative number of stars ejected by the supermassive black holes of the LMC and the Milky Way, along with their speeds, the team estimated that the LMC’s black hole has a mass of approximately 600,000 times that of the Sun.
In contrast, the Milky Way's supermassive black hole has roughly 4 million solar masses. Supermassive black holes with billions of times the mass of the Sun can be found elsewhere in the universe.