First dormant stellar-mass black hole discovered outside of our galaxy

Artist’s rendering of a black hole orbiting a large hot blue star. Photo credit: ESO/L. Calçada

Detecting black holes is not easy. By definition they are black because nothing can escape them, not even light. So astronomers have to find other ways to find them. Gravitational waves, when they merge, are one way. Or by strong X-rays when eating. This means that finding the silent, the so-called dormant black holes is extremely difficult. Finding one now in the galaxy next door is certainly cause for celebration then.

The team of astronomers viewed nearly 1,000 stars in the Tarantula Nebula, located in the Large Magellanic Cloud, the Milky Way’s largest galactic companion. Among those objects was VFTS 243, whose observations are consistent with a black hole nine times the mass of our Sun orbiting a hot blue star nearly three times its mass.

The finding, reported in Nature Astronomy, comes from a team that has spent several years disproving previous dormant black hole candidates. So they approached what they saw with a grain of salt. Finally, there could be other explanations that fit the observations when you’re about 160,000 light-years away.

“As a researcher who has debunked potential black holes for the last few years, I was extremely skeptical about this discovery,” study leader Tomer Shenar of the University of Amsterdam said in a statement.

“When Tomer asked me to double-check his results, I had my doubts. But I couldn’t find a plausible explanation for the data that didn’t include a black hole,” explained co-author Kareem El-Badry of the Center for Astrophysics | Harvard & Smithsonian.

Stellar-mass black holes like this one form at the end of a massive star’s life. Many will form after the star goes supernova and eject some of the stellar material into interstellar space. But not this one. The team believes this was a special formation.

“The star that formed the black hole in VFTS 243 appears to have collapsed completely, with no evidence of a previous explosion,” Shenar added. “Evidence for this ‘direct collapse’ scenario has surfaced recently, but our study arguably provides one of the most direct clues. This has tremendous implications for the origin of black hole mergers in the cosmos.”

The team invites other groups to verify their findings, as it is only fair after spending so many years as ‘black hole police’. They hope that once VFTS 243 is confirmed, their approach will lead to the discovery of many more of these objects.

“It’s incredible that we hardly know of any dormant black holes, given how common astronomers assume they occur,” said co-author Pablo Marchant from KU Leuven.

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