A sleeping black hole discovered outside our galaxy

O stellar mass black holes they form when massive stars reach the end of their lives and collapse under their own gravity. On a Binary System (a system of two stars revolving around each other), this process leaves a black hole in orbit with a luminous companion star.

Sometimes this kind of black hole remains ‘inactive’, it does not emit the usual high levels of X-ray radiation, which is how they are normally detected. Now one has been found outside the Milky Way.

An international team of astronomers known as ‘the black hole police‘, which refuted several findings of this type of object, detected an inactive stellar-mass black hole in the Large Magellanic Cloud, a neighboring galaxy to ours. Furthermore, the star that gave rise to this hole disappeared without any sign of a powerful explosion, as reported in the journal Nature astronomy.

“For the first time, our team came together to publicize the discovery of a black hole rather than disprove it; we identified a needle in a haystack”, emphasizes the main author, Tomer Shenarwho started studying at the KU Leuven center (Belgium) and now has a Marie-Curie fellowship at the University of Amsterdam (Netherlands).

While other black hole-like candidates have been proposed, the team claims that this is the first inactive stellar-mass black hole to be unequivocally detected outside our galaxy. It is part of VFTS 243 binary systemcomposed of a hot blue star with 25 times the mass of the Sun and a black hole, which is at least nine times the mass of the Sun. The star is about 200,000 times larger than the hole.

“It’s amazing that we barely know about the existence of these dormant black holes, given how common the astronomical community assumes they are,” explains the co-author, Paul Marchant, by KU Leuven. Sleeping black holes are particularly difficult to detect as they don’t interact much with their environment.

“For more than two years, we’ve been looking for these types of binary black hole systems,” adds the co-author, Julia Bodensteinerresearcher at the European Southern Observatory (ESO) in Germany, who was excited when she learned of the VFTS 243 data, “which in my opinion is the most convincing candidate reported to date”.

In the Tarantula Nebula

To find this binary system, the collaboration searched almost 1000 stars mass in the Tarantula Nebula region of the Large Magellanic Cloud, focusing on those that may have black holes as companions. Identifying them as black holes is extremely difficult as there are many alternative possibilities.

The study focused on the Tarantula Nebula, the most prominent object in the Large Magellanic Cloud, a satellite galaxy of our Milky Way. / THAT

“As a researcher who has disproved possible black holes in recent years, I was extremely skeptical of this discovery,” insists Shenar. Skepticism was shared by co-author Karim El Badry, from the Harvard & Smithsonian Center for Astrophysics (USA), whom Shenar calls a “black hole destroyer”. “When Tomer asked me to review his findings, I had my doubts. But I couldn’t find a plausible explanation for the data that didn’t involve a black hole,” explains El-Badry.

The discovery also gives the team unique insight into the processes that accompany the formation of black holes. The astronomical community believes that a stellar-mass black hole forms when the core of a dying massive star collapses, but it remains unclear whether this process is accompanied by a powerful supernova explosion.

“The star that formed the black hole in VFTS 243 appears to have completely collapsed, with no signs of a previous explosion,” says Shenar, who recalls that evidence of “this ‘direct collapse’ scenario emerged recently, but our study undoubtedly provides one of the clearest indications; And that has huge implications for the origin of black hole mergers in the cosmos.”

Six years of observations with the VLT

The black hole in VFTS 243 was found using six years of observations of the Tarantula Nebula carried out by the instrument. FLAMES (Fiber Large Array Multi Element Spectrograph), installed in the Very Large Telescope (VLT) of ESO in Chile.

Despite the nickname ‘black hole police’, the team actively encourages scrutiny, debate with alternative models and hopes their work will lead to the discovery of other stellar-mass holes orbiting massive stars, thousands of which are predicted. Path and in the Magellanic Clouds.