Dormant black hole discovered outside our galaxy

A team of international experts, renowned for debunking several black hole discoveries, has now found a stellar-mass black hole in the Large Magellanic Cloud, a neighbour galaxy to our own. Moreover, they found that the star that gave rise to the black hole vanished without any sign of a powerful explosion.

While similar black hole candidates have been proposed, the team claims this is the first ‘dormant’ stellar-mass black hole to be unambiguously detected outside our galaxy. Their work has been published in the journal Nature Astronomy.

Stellar-mass black holes are formed when massive stars reach the end of their lives and collapse under their own gravity. In a binary, a system of two stars revolving around each other, this process leaves behind a black hole in orbit with a luminous companion star. The black hole is ‘dormant’ if it does not emit high levels of X-ray radiation, which is how such black holes are typically detected. But although astronomers believe dormant black holes to be quite common, they are particularly hard to spot since they do not interact much with their surroundings.

“We identified a needle in a haystack,” said study leader Tomer Shenar, who started the study at KU Leuven in Belgium and is now based at the University of Amsterdam.

To find the binary system VFTS 243, the researchers searched nearly 1000 massive stars in the Tarantula Nebula region of the Large Magellanic Cloud using the European Southern Observatory’s (ESO) Very Large Telescope (VLT), looking for the ones that could have black holes as companions. Identifying these companions as black holes is extremely difficult, as so many alternative possibilities exist.

The black hole was eventually found using six years of observations of the Tarantula Nebula by the VLT’s Fibre Large Array Multi Element Spectrograph (FLAMES) instrument. It was found to be at least nine times the mass of our Sun, and orbits a hot, blue star weighing 25 times the Sun’s mass.

This animation shows what the binary system VFTS 243 might look like if we were observing it up close and at an inclination different from that we see from Earth. Courtesy of ESO/L Calçada under CC BY 4.0

The discovery allows the team a unique view into the processes that accompany the formation of black holes. Astronomers believe that a stellar-mass black hole forms as the core of a dying massive star collapses, but it remains 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,” Shenar said. “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.”

The team hopes that their work will enable the discovery of other stellar-mass black holes orbiting massive stars, thousands of which are predicted to exist in Milky Way and in the Magellanic Clouds.

Image caption: Artist’s impression shows what the binary system VFTS 243 might look like if we were observing it up close. The sizes of the two binary components are not to scale: in reality, the blue star is about 200,000 times larger than the black hole. Courtesy of ESO/L Calçada under CC BY 4.0

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