A small black hole that is spinning around, quietly minding its own business, has been detected outside the Milky Way galaxy.
This, the team says, marks the first time we have been able to conclusively detect and identify an inactive black hole that is not within the confines of our own galaxy.
Although a similar discovery was announced last year, the evidence of his true identity left some room for doubt. This most recent finding, according to the authors, is a solid bet.
It is true that the object is not so far from the Milky Way, located within a satellite galaxy called the Large Magellanic Cloud. But the discovery is one that could help us find more such black holes in the future, and it has implications for our understanding of black hole formation to begin with.
The team behind the report is clearly excited, not least because of their skeptical reputation for casting doubt on previous findings of dormant black holes.
“For the first time, our team came together to report the discovery of a black hole, rather than reject it,” said astronomer Tomer Shenar of the University of Amsterdam in the Netherlands.
“We identified a ‘needle in a haystack’.”
Black holes are tricky little beasts. Its extreme density produces an extreme gravitational field which means that not even light whizzing through a vacuum, the fastest anything in the Universe can go, can reach escape velocity.
This means that they are cloaked in darkness and do not emit any light that we can detect. The exception is when they are actively “feeding” or accumulating material. This is a complicated process that produces a telltale signature of X-ray radiation from the neighborhood immediately around the black hole.
Dormant or dormant black holes are more or less invisible. But that pesky gravity can give it away…if you know how to look. If a stellar-mass black hole is in a binary system with another star, the orbital motion of the companion surrounding what appears to be empty space could indicate the presence of the black hole.
However, not all dark masses are black holes. Other astronomers have been fooled before, the most famous example being a black hole touted as the closest to Earth ever found. A small, dim companion whose light cannot be discerned could be the culprit, meaning no option can be left unconsidered.
Shenar and members of his team, including astronomers Kareem El-Badry of the Harvard & Smithsonian Center for Astrophysics and Julia Bodensteiner of the European Southern Observatory, have been among those who have thoroughly debunked such discoveries. But that doesn’t mean they think such black holes don’t exist; only that the evidence must be irrefutable.
“For more than two years, we have been searching for such binary black hole systems,” Bodensteiner explained.
The focus of their search was the Tarantula Nebula in the Large Magellanic Cloud, a stellar nursery where extremely massive young stars can be found. The researchers studied about 1,000 of these massive young stars in detail, looking for the telltale wobble of a binary orbit.
When two objects orbit each other, they do so around a mutual center of gravity called the barycenter. For a system like the Earth and the Sun, the barycenter would be so close to the center of the Sun that it would be difficult to see the star move around it from a great distance. If the Earth were more massive, the Sun’s spin around the barycenter would be much easier to detect.
We can detect this wobbling motion, or radial velocity, in the object’s light spectrum as it stretches into longer (redder) wavelengths away from us and squeezes into shorter (bluer) wavelengths moving toward us. U.S.
The team looked for these spectral changes in their sample and hit the mark: a massive blue-white O-type star 25 times the mass of the Sun, about 160,000 light-years away. When they calculated the mass of the object that could cause the observed wobble, they found that the companion was about 9 times the mass of the Sun. At that mass, the black hole’s event horizon would be about 27 kilometers (17 miles) wide.
And yet it was invisible. The upper mass limit for neutron stars is around 2.3 times that of the Sun, so that rules them out. Other stars in their sample that wobbled were ruled out using the team’s techniques for detecting light from faint companion stars and modeling the expected light from a faint companion on the observed mass.
None of the alternative explanations fit the observational data.
“When Tomer asked me to double check his findings, I had my doubts,” El-Badry said. “But I couldn’t find a plausible explanation for the data that didn’t involve a black hole.”
The binary, called VFTS 243, could hold important clues about how black holes form. Scientists believe there are multiple scenarios. The first is a colossal supernova in which an unstable star erupts, blasting its outer material into space while the core collapses into a black hole, born of fire and fury.
The second is a direct collapse in which a dying star, no longer supported by the external pressure provided by atomic fusion, simply… collapses in on itself, not with a bang but with a silent ker-floomp.
“The star that formed the black hole in VFTS 243 appears to have completely collapsed, with no sign of a previous explosion,” Shenar said. “Evidence for this ‘direct collapse’ scenario has emerged only recently, but our study arguably provides one of the most direct indications. This has huge implications for the origin of black hole mergers in the cosmos.”
Although, of course, having examined so many other black hole discoveries, the team now invites other astronomers to examine theirs. Fair fair.
The research has been published in nature astronomy.
