A dormant black hole at least nine times the mass of the our Sun has been detected just 160,000 light years from Earth. It’s the first time one has been found “unambiguously” outside our galaxy. This sleeping giant is not currently devouring gas or other matter, astronomers say.
The dormant black hole lies in the Large Magellanic Cloud, a satellite galaxy that neighbors the Milky Way and orbits a hot, blue star that is almost three times as big.
Astronomers liken the discovery to finding a “needle in a haystack.”
Dormant black holes are particularly hard to spot since they do not interact with their surroundings.
“For more than two years now we’ve been looking for such black-hole-binary systems. I was very excited when I heard about VFTS 243, which in my opinion is the most convincing candidate reported to date,” adds co-author Dr. Julia Bodensteiner, of the European Southern Observatory (ESO) in Germany.
The star that gave rise to it vanished without any sign of a powerful explosion.
The international team – dubbed the “black hole police” – searched nearly 1,000 massive stars throughout the spectacular Tarantula Nebula in the constellation of Dorado. Identifying companions as black holes is extremely difficult as so many alternative possibilities exist.
“As a researcher who has debunked potential black holes in recent years, I was extremely skeptical regarding this discovery,” says research leader Tomer Shenar who started the study at KU Leuven, and is now a Marie-Curie Fellow at Amsterdam University. “For the first time, our team got together to report on a black hole discovery – instead of rejecting one.”
Colleagues included Dr. Kareem El-Badry of Harvard University in Boston, who is nicknamed “black hole destroyer” by his peers. “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,” admits El-Badry.
The study also sheds light on how black holes are created from the cores of dying stars. It was uncertain whether or not this is accompanied by a powerful supernova explosion.
“The star that formed the black hole in VFTS 243 appears to have collapsed entirely – with no sign of a previous explosion,” explains Shenar. “Evidence for this ‘direct-collapse’ scenario has been emerging recently – but our study arguably provides one of the most direct indications. This has enormous implications for the origin of black-hole mergers in the cosmos.”
Stellar-mass black holes form when massive stars reach the end of their lives and collapse under their own gravity. In a binary system – two stars revolving around each other – this process leaves behind a black hole in orbit with a luminous companion. Black holes are “dormant” if they do not emit high levels of X-ray radiation, which is how they are typically detected. Identification of VFTS 243 was made thanks to six years of data obtained with ESO’s Very Large Telescope (VLT).
The FLAMES (Fibre Large Array Multi Element Spectrograph) scanner allows more than a hundred objects to be observed at once, a significant saving of time.
Despite the nickname “black hole police,” they actively encourages scrutiny. Thousands of stellar-mass black holes are believed to exist in the Milky Way and the Magellanic Clouds. They are much smaller than the supermassive black hole 27,000 light years from Earth that is powering the Milky Way.
“Of course I expect others in the field to pore over our analysis carefully, and to try to cook up alternative models. It’s a very exciting project to be involved in,” notes El-Badry.
The research is published in the journal Nature Astronomy.
Report by South West News Service writer Mark Waghorn