They discover an inactive black hole in a nearby galaxy

An international team of researchers has discovered 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, at least nine solar masses in size, was caused by a hot blue star 25 times the mass of the Sun that disappeared with no sign 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 candidate black holes have been proposed, the team says this is the first “dormant” stellar-mass black hole to be unambiguously detected outside our galaxy. Baptized VFTS 243, it was released this Monday in the magazine “Natural 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 rotating around each other, this process leaves a black hole orbiting with a bright companion star. The black hole is “dormant” if it does not emit high levels of X-ray radiation, which is how these phenomena are usually detected.

“It’s amazing that we barely know about dormant black holes, given how much astronomers think about them,” says co-author Pablo Marchant of KU Leuven. The truth is that these holes are particularly difficult to detect because they don’t interact much with their surroundings.

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”, explains co-author Julia Bodensteiner, ESO researcher in Germany. “I was very excited when I heard about VFTS 243, which I believe 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 region of tarantula nebula of Large Magellanic Cloud, trying to identify those that might have black holes as companions. Identifying these companions as black holes is extremely difficult, as there are many alternative possibilities.

“Black Hole Destroyer”

The finding is all the more curious as these researchers are particularly known for debunking other black hole discoveries. “As a researcher who has debunked potential black holes for the past few 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), which 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 implicate a black hole,” says El-Badry.

The discovery also gives the team unique insight into the processes that accompany black hole formation. Astronomers believe a stellar-mass black hole forms when the core of a dying massive star collapses, but it’s 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 signs of a previous explosion,” Shenar said. “Evidence for this ‘direct collapse’ scenario has only recently emerged, but our study provides arguably one of the most direct indications. This has huge implications for the origin of black hole mergers in the world. cosmos.”

The black hole in VFTS 243 was discovered 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 a time, a significant time saving at the telescope compared to studying each object one by one.

Despite the nickname ‘black hole police’, the team actively encourages scrutiny and hopes their work will lead to the discovery of other stellar-mass black holes orbiting massive stars, thousands of which are thought to exist in the Milky Way and Magellanic Clouds .

And no one is immune to scrutiny. “Of course, I hope others in the field will look carefully at our analysis and try to come up with alternative models,” El-Badry says sportingly.

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