An international team of researchers has found a dormant black hole in the Large Magellanic Cloud, a neighboring galaxy to our Milky Way, about 160,000 light-years away. The black hole, of at least nine solar masses, was caused by a hot blue star 25 times the mass of the Sun that disappeared without signs of having caused a powerful explosion.
“We identified a ‘needle in a haystack,'” says Tomer Shenar of the University of Amsterdam in the Netherlands. Although other similar black hole candidates have been proposed, the team says this is the first ‘dormant’ stellar-mass black hole to be unambiguously detected outside our galaxy. Called VFTS 243, it has been released this Monday in the journal ‘Nature Astronomy’.
Stellar-mass black holes form when massive stars reach the end of their lives and collapse under their own gravity. In a binary system, a system of two stars revolving around each other, this process leaves an orbiting black hole with a luminous companion star. The black hole is ‘dormant’ if it is not emitting high levels of X-ray radiation, which is how these phenomena are typically detected.
“It’s amazing that we hardly know of dormant black holes, given how common astronomers think they are,” explains co-author Pablo Marchant of KU Leuven. The truth is that these holes are particularly difficult to detect since they do not interact much with their environment.
The discovery was made thanks to six years of observations obtained with the Very Large Telescope (VLT) of the European Southern Observatory (ESO). “For more than two years, we have been looking for these types of binary black hole systems,” says the co-author Julia Bodensteiner, ESO researcher in Germany. “I was very excited when I heard about VFTS 243, which in my opinion is the most compelling candidate reported to date,” she says.
To find VFTS 243, the collaboration searched for nearly 1,000 massive stars in the Tarantula Nebula region of the Large Magellanic Cloud, trying to identify those that might have black hole companions. Identifying these companions as black holes is extremely difficult, as there are so many alternative possibilities.
‘Black hole destroyer’
The finding is even more curious considering that these researchers are especially known for debunking other black hole discoveries. “As a researcher who has debunked potential black holes in recent years, I was extremely skeptical of this discovery,” admits Shenar.
The skepticism was shared by co-author Kareem El-Badry of the Center for Astrophysics | Harvard & Smithsonian (CfA), whom Shenar calls the “black hole destroyer” for all the candidates he has shot down. “When Tomer asked me to double check his findings, I had my doubts. But I couldn’t find a plausible explanation for the data that didn’t involve a black hole,” explains El-Badry.
The discovery also allows the team a unique insight into the processes that accompany the formation of black holes. Astronomers believe that a stellar-mass black hole forms when the core of a dying massive star collapses, but it remains unclear whether or not this is accompanied by a powerful supernova explosion.
“The star that formed the black hole in VFTS 243 appears to have completely collapsed, with no sign of a previous explosion,” Shenar says. “Evidence for this ‘direct collapse’ scenario has only recently emerged, 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.”
The black hole in VFTS 243 was found after six years of observations of the Tarantula Nebula by the spectrograph (FLAMES) on ESO’s VLT. FLAMES allows astronomers to observe more than a hundred objects at once, a significant saving in telescope time compared to studying each object one by one.
Despite the nickname ‘black hole police’, the team actively encourages scrutiny and hopes that their work will lead to the discovery of other stellar-mass black holes orbiting massive stars, thousands of which are predicted to exist in the Milky Way and Magellanic Clouds.
And no one is exempt from scrutiny. “Of course, I hope that others in the field will take a hard look at our analysis and try to come up with alternative models,” says El-Badry sportingly.