Dormant black hole discovered outside Milky Way for the first time; here's all about it

Astronomers have identified a ‘needle in a haystack’ in form of a black hole lurking outside the Milky Way. Measuring about nine times the mass of the sun, this new black hole has been labeled as the first ‘dormant’ stellar-mass black hole to be unambiguously detected outside our galaxy. While this type of black hole is extremely common, they are extremely difficult to spot because they do not interact much with their surrounding. 

Published in the journal Nature Astronomy, the findings suggest that this dormant black hole was found after six years of observations by the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in the Large Magellanic Cloud, a neighbour galaxy. "For the first time, our team got together to report on a black hole discovery, instead of rejecting one," lead study author Tomer Shenar said in an official statement. 

Discovering the rare black hole

This dormant black hole is said to have been born out of a star that vanished without any sign of a powerful explosion. According to Shenar and the team, it is about nine times more massive than the sun and orbits a hot, blue star weighing 25 times greater than the sun’s mass. The two entities make up a binary system which has been named VFTS 243.

Notably, the new discovery is a stellar-mass black hole, the kind which forms when massive stars reach the end of their lives and collapse under their own gravity. And the case as the VFTS 243 emerges when one of the two stars in the system turns into a black hole and keeps orbiting the other luminous star. 

(The region around Tarantula Nebula; Image: ESO)

"It is incredible that we hardly know of any dormant black holes, given how common astronomers believe them to be", co-author Pablo Marchant said. To find the binary system VFTS 243, the astronomers observed nearly 1000 massive stars in the Tarantula Nebula region of the Large Magellanic Cloud.

"The star that formed the black hole in VFTS 243 appears to have collapsed entirely, with no sign of a previous explosion," Shenar said. "Evidence for this ‘direct-collapse’ scenario has been emerging recently, but our study arguably provides one of the most direct indications". Interestingly, the astronomers involved in this study believe that this discovery would have enormous implications for the origin of black-hole mergers in the cosmos. They also say that it would enable the discovery of other stellar-mass black holes orbiting massive stars, thousands of which are predicted to exist in our galaxy and its neighbour.