Hidden black hole found in extragalactic star cluster

Astronomers have just unveiled a black hole from another galaxy, hiding deep within a bright cluster of a thousand stars. The black hole doesn’t emit any light visible to telescopes; its presence was betrayed by its gravitational influence on the nearby stars.

This marks the first time that a black hole has been spotted outside of the Milky Way using this detection method.

Astronomer Sara Saracino, from Liverpool John Moores University in the UK, says that when they found a star in this cluster behaving weirdly, it was like a “smoking gun”.

“Similar to Sherlock Holmes tracking down a criminal gang from their missteps, we are looking at every single star in this cluster with a magnifying glass in one hand trying to find some evidence for the presence of black holes, but without seeing them directly,” she explains.

“The result shown here represents just one of the wanted criminals, but when you have found one, you are well on your way to discovering many others, in different clusters.”

Saracino was lead author on the study, published in Monthly Notices of the Royal Astronomical Society.

About 11 times the mass of our Sun, this small black hole was found within the star cluster NGC 1850, in a neighbouring galaxy called the Large Magellanic Cloud.

Previously, stellar-mass black holes like this one have only been detected beyond our galaxy via other methods: either the X-ray glow they emit as they superheat gas and other material, or by the gravitational waves that cascade out when they collide with another massive object, like a neutron star.

But this detection method – looking at the subtle but detectable gravitational influence on nearby stars – has never been used to find a black hole from another galaxy.

It was, however, the method used to confirm the supermassive black hole Sagittarius A* at the heart of our own galaxy – over decades, astronomers watched how the gravity of the black hole forced the stars to dance in odd orbits, like puppets on a string.

An animation of the orbits of stars and gas clouds near the supermassive black hole at the centre of our galaxy. Credit: Advanced Visualisation Lab, National Centre for Supercomputing Applications, University of Illinois.

Applying this method outside of the Milky Way may allow astronomers to find many more black holes.

“Every single detection we make will be important for our future understanding of stellar clusters and the black holes in them,” says study co-author Mark Gieles, from the University of Barcelona, Spain.

It would also shed light on the evolution of stellar-mass black holes, including how they grow by feeding on stars or merging with other black holes.

The discovery was made possible by MUSE – a powerful instrument on the Very Large Telescope (VLT) in Chile, operated by the European Southern Observatory.

MUSE (the Multi Unit Spectroscopic Explorer) is an integral field spectrograph, which can produce 3D datasets of an object, where each pixel of the image contains information about the full spectrum of light.

“MUSE allowed us to observe very crowded areas, like the innermost regions of stellar clusters, analysing the light of every single star in the vicinity,” explains co-author Sebastian Kamann, also from Liverpool’s Astrophysics Research Institute.

“The net result is information about thousands of stars in one shot, at least 10 times more than with any other instrument.”

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