ʻBlack hole guardsʼ find a dormant Black hole outside our galaxy

“For the first time, our team got together to report on a black hole discovery instead of rejecting one”, notes the director of the study, Tomer Shenar. The team found that the star that gave rise to the black hole vanished without any sign of any powerful explosion. This finding has been possible thanks to six years of observations obtained with the Very Large Telescope (VLT) of the European Southern Observatory (ESO). The study, carried out by nearly forty international experts, includes the participation of members of the Institute of Astrophysics of the Canary Islands (IAC) and the University of La Laguna (ULL).

“We found a needle in a haystack”, says Shenar, who began the study at KU Leuven (Belgium) and is now a Marie-Curie fellow at Amsterdam University (the Netherlands). Although other similar black holes have been proposed, the team says this is the first dormant stellar-mass black hole unambiguously detected outside our galaxy. Stellar-mass black holes form when massive stars reach the end of their lives and collapse under their own gravity.

In a binary system, formed by two stars that revolve around each other, this process leaves a black hole in orbit with a luminous companion star. “From the nearly circular orbit of these objects, we could conclude that this black hole did not receive a velocity kick when it formed in a supernova explosion, which will help us disclose the origin of gravitational waves detected by the LIGO-Virgo detectors”, says ICREA professor Mark Gieles, member of the Department of Quantum Physics and Astrophysics of the UB, who analysed and interpreted the number of detected compact object collisions, such as binary systems that contain black holes.

The black hole is dormant if it does not emit high levels of X-rays —the way in which these black holes are detected. “It is incredibly that we hardly know of any dormant black hole, given how common astronomers believe them to be”, says Pablo Marchant, co-author of the study and member of KU Leuven. The found black hole is at least nine times the mass of our Sun, and it orbits a hot blue star which weighs twenty-five times the mass of the Sun.

Dormant black holes are especially hard to detect, since they do not interact much with their surroundings. “For more than two years now, we have been looking for such black hole binary systems”, notes the co-author of the study, Julia Bodensteiner, researcher at the European Southern Observatory (ESO) in Germany. “I was very excited when I heard about VFTS 243 —she adds— which in my opinion is the most convincing candidate reported to date”.

To find the VFTS 243, the collaboration analysed about 1,000 massive stars in the Tarantula Nebula region of the Large Magellanic Cloud to find those that could have black holes as companions. Identifying these companions as black holes is difficult since there are many alternative options.

“As a researcher who has debunked potential black holes in recent years, I was very sceptical regrading this discovery”, says Shenar. This scepticism was shared by the co-author of the study, Kareem El-Badry, member of the Center for Astrophysics | Harvard & Smithsonian (United States), whom Shenar calls “the black hole destroyer”. “When Tomer asked me to double check his findings, I had my doubts. But I could not find a plausible explanation for the data that did not involve a black hole”, says El-Badry.

The findings allow the team to have a unique view into the processes that accompany the formation of black holes. Astronomers believe that a stellar-mass black hole forms when the nucleus of a dying massive star collapses, but whether this goes together with a supernova explosion remains unknown.

“The star that formed the black hole in VFTS 243 appears to have collapsed entirely, with no sign of a previous explosion”, says Shenar. “Evidence for this direct-collapse scenario has been emerging recently but out study arguably provides one of the most direct indications. This has enormous implications for the origin of black-hole mergers in the cosmos”, adds the expert.

The black hole in VFTS 243 was found thanks to six years of observations of the Tarantula Nebula by the Fibre Large Array Multi Element Spectograph (FLAMES) on ESOʼS VLT.

Despite the nickname of “black hole police”, the team actively promotes scrutinity, and they hope their work to enable the discovery of other stellar-mass black holes orbiting massive stars, thousands of which are predicted to exist in the Milky Way and in the Magellanic Clouds.

“I expect other experts to pore over our analysis carefully and try to cook up alternative models”, concludes El-Badry. “It is a very exciting project to be involved in”.

Reference article:

Shenar, T. et al. «An X-ray-quiet black hole born with a negligible kick in a massive binary within the Large Magellanic Cloud». Nature Astronomy, July, 2022. DOI: 10.1038/s41550-022-01730-y