For the first time, astronomers have confirmed the existence of a lone black hole. This rogue is wandering around the Milky Way — our galaxy — with no companion star.
It’s “the only [loner] so far,” says Kailash Sahu. He’s an astronomer at the Space Telescope Science Institute in Baltimore, Md.
In 2022, Sahu was part of a team that found the dark object moving through the constellation Sagittarius. (This group of stars in the southern sky takes its name from the Latin for “archer.”)
At the time, this discovery made headlines. The reason: Until then, all known black holes had a companion star. Those companions had pointed to the presence of the black holes (which can’t be seen because they emit no light).
A second team would go on to dispute the claim that this rogue was a black hole. They argued it could be a neutron star.
But new observations by the Hubble Space Telescope now confirm the rogue object is about seven times as massive as the sun. That’s so massive that the dark object is a black hole. Black holes are more than three times as massive as the sun. Neutron stars, though, have less mass than this. So the dark object must be a black hole, Sahu’s group reasoned.
The team shared that finding in the April 20 issue of The Astrophysical Journal.
Acting as a ‘lens’
Solitary black holes should be common. But since they don’t normally affect visible stars, they’re hard to find. The lone black hole in Sagittarius revealed itself only because it passed in front of a dim background star.
During that passage in July 2011, the black hole’s intense gravity magnified the star’s light through what’s known as gravitational lensing. This lensing slowly shifted the star’s apparent position, too.
That star’s position still hasn’t settled down. This shows why “it takes a long time to do the observations,” Sahu says.
The rogue’s original discovery relied on Hubble’s precise measurements of star positions from 2011 to 2017. The new study adds in Hubble data from 2021 and 2022. It also uses data collected by the European Space Agency’s Gaia spacecraft.
What these data now show: The black hole is 7.1 times as massive as the sun (give or take 0.8 solar mass).
And that research team of doubters? They revised their assessment in 2023. They now agree the object is a black hole. That team estimates the object’s mass may be only six times that of the sun. But because this team is not as certain of the black hole’s mass as Sahu’s group, the two results are considered consistent.
Located 5,000 light-years from Earth, the newfound rogue is much closer than the supermassive black hole at our Milky Way’s center. That central black hole also lies in Sagittarius. It’s also more than five times farther from us.
The star-rich region around the galactic center offers a great hunting ground for other rogue black holes passing in front of stars. Sahu hopes to find additional such loners by using the Nancy Grace Roman Space Telescope. It’s due to launch in 2027.