Mysterious “Dormant” Black Hole Discovered in Deep Space

The Large Magellanic Cloud, a neighboring galaxy to the Milky Way, is home to a dormant stellar-mass black hole, according to a group of researchers who is known for disproving discoveries of Black holes in the past.

“For the first time, our team came together to report the discovery of a black hole, rather than reject one,” explains study leader Tomer Shenar, a Marie-Curie fellow at the University of Amsterdam, the Netherlands.

Scientists from the Center for Astrophysics/Harvard and Smithsonian (CfA) in the United States, including Kareem El-Badry, found that the star that gave rise to the black hole vanished most liklely without any signs of an explosion.

“We have identified a needle in a haystack,” Shenar said. Even though other similar black hole candidates have been proposed, this is the first unambiguously detected stellar-mass black hole outside the Milky Way.

Massive stars collapse under their own gravity when they reach the end of their lives and form stellar-mass black holes. When two stars orbit each other, they create a binary system with a black hole orbiting the bright companion star. If a black hole does not emit high levels of X-ray radiation, which is how they are usually detected, then it is considered “dormant.”

A six-year observation campaign carried out by the European Southern Observatory (ESO) led to the discovery.

“It’s amazing that we hardly know of any dormant black holes, considering how common astronomers think they are,” admits co-author Pablo Marchant of the University of Leuven (KU Leuven) in Belgium.

In fact, the newly discovered black hole is about nine times the mass of the Sun and orbits a hot, blue star about 25 times the mass of our Sun. In other words, the star around which the “dormant” Black Hole orbits is roughly 200,000 times larger than the black hole itself.

The collaboration found VFTS 243 by searching for nearly 1,000 massive stars in the Tarantula Nebula region of the Large Magellanic Cloud. This was done to determine if they had black hole companions. Unfortunately, many alternative possibilities may be present, making identifying these companions as black holes difficult.

The first image of the supermassive black hole in the center of the Milky Way - Sagittarius A*. Credit: ESA
The first image of the supermassive black hole in the center of the Milky Way – Sagittarius A*. Credit: ESA

“As a researcher who has debunked possible black holes in recent years, I was extremely skeptical of this discovery,” admits Shenar.

“When Tomer asked me to check his conclusions, 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.

Also, the discovery provides unique insight into the processes associated with black hole formation. Black holes with stellar mass are believed to form when the core of a dying mass star collapses. However, it is not known whether a powerful supernova explosion may accompany this collapse.

In the animation above, we see how the two cosmic objects of VFTS 243 orbit one another. In reality, the star is roughly 200,000 times larger than the black hole, based on data obtained by scientists. Credit: ESO/L. Calçada.

VFTS 243’s black hole is believed to have been formed as a result of the collapse of the star, although there is no sign of a previous explosion, according to Shenar. “Evidence for this ‘direct collapse’ scenario has recently emerged, but our study possibly provides one of the most direct indications. This has huge implications for the origin of black hole mergers in the cosmos.”

ESO’s VLT using the FLAMES instrument (Fiber Large Array Multi Element Spectrograph), discovered the black hole in VFTS 243 after six years of observing the Tarantula Nebula. FLAMES allows astronomers to study many objects at the same time, saving valuable telescope time.

The team, although called the “black hole police,” actively encourages scrutiny and hopes that their work in Nature Astronomy (July 18, 2022) will lead to the discovery of other stellar-mass black holes orbiting massive stars, which are estimated to number in the thousands in the Milky Way and in the Magellanic Clouds.

“Of course, I hope other experts will take a serious look at our analysis and try to come up with alternative models,” says El-Badry. “It’s a very exciting project to be involved in.”

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