Lone black holes probably litter the galaxy, but they are extremely difficult to detect. Now, astronomers have seen, for the first time, an isolated black hole, wandering alone in the Milky Way. Black holes are usually glimpsed when they interact with other objects, such as companion stars. Studying black holes alone, a separate class of cosmic object, should help scientists understand how they form and how abundant they are.
Black holes are so massive that not even light can escape their gravitational pull, so by their nature they are invisible. They remain ghostly unless they interact with other stellar objects, for example by superheating matter from a companion star, or colliding to generate gravitational waves that spread throughout the Universe. Lone black holes are thought to be common, forming every time a single star of more than 20 solar masses reaches the end of its life. “There should be 100 million such black holes in the galaxy, they should be everywhere, but it’s very hard to find them,” says Kailash Sahu, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, who led the team that made the discovery. Confirming the black hole’s presence took ten years of observations, using NASA’s Hubble Space Telescope and several ground-based observatories. A preliminary paper 1 detailing the find, published on arXiv on January 31, has yet to be peer-reviewed, but is already getting astronomers excited. “I think this is a very exciting and important discovery,” says Selma de Mink, an astrophysicist at the Max Planck Institute for Astrophysics in Garching, Germany.
To detect the lone black hole, the international team used a technique known as microlensing. The researchers looked for stars that appeared to glow when an invisible object passed by, its gravitational pull bending and focusing starlight like a lens (see ‘Cosmic Lenses’). Very massive objects, such as black holes, affect a larger area of space and thus illuminate the star for a longer time. But fainter, lighter objects like neutron stars that move unusually slowly could also cause a prolonged glow.
The team then selected eight candidate objects, which did not produce their own light but illuminated a background star for at least 200 days, for further observations. They now have enough data to say that one is a black hole. For six years, the group used the Hubble Space Telescope to measure how the passing object appeared to shift a star’s position in the sky. This deviation is miniscule: measuring it from Earth is equivalent to someone in New York measuring the width of a coin in Los Angeles, says Sahu. Using equations first derived by Albert Einstein in 1915, the researchers were able to calculate that the invisible object’s mass is about seven times that of the Sun: heavy enough that it is almost certainly a black hole. “They were the first to unambiguously detect a single black hole,” says de Mink. The detection is much more convincing than previous claims of lone black holes, agrees Eric Agol, an astronomer at the University of Washington in Seattle, who 20 years ago was among the researchers who proposed the combination of illumination and diversion techniques .
Finally, additional information came from ground-based observatories that saw the brightening event. Slight differences in the angle at which light hits various places on the globe created a parallax effect that identified the black hole’s distance as 1.58 kiloparsecs (5,150 light-years) from Earth.
Putting the distance and mass of the object together with the duration of the brightness revealed that the black hole is traveling across our field of view at about 45 kilometers per second. This is slightly faster than the 10 to 30 kilometer per second speeds of other stars in its neighborhood, says Sahu. That could indicate that the black hole got an extra “kick” when it formed in the core of a supernova explosion. “What a way to be born,” he says. This measurement is exciting because the kicks black holes receive at birth are still the subject of “great debate” and have implications for understanding supernovae, says Ilya Mandel, an astronomer at Monash University. in Melbourne, Australia. But he points out that astronomers know the speed of the black hole in only one direction; It could still lag behind its neighbors if it’s moving more slowly than they do in another direction, such as away from Earth. The work is “a very good result that demonstrates an exciting technique, but many questions remain,” he says.
Although little can be learned from a single example, the team has three other promising black hole candidates to examine. As more findings emerge, they could help astronomers discover much more about the origins of isolated black holes, as well as how common they are, says Feryal Özel, an astronomer at the University of Arizona in Tucson. “We shouldn’t infer anything from a single data point, but I’m excited,” he says.