Black hole found hiding in star cluster outside our galaxy

Artist's impression of a black hole that is 11 times larger than the Sun and five times more massive than the star with five solar-mass orbiting it. These two objects are found in NGC1850, a cluster of thousands stars located approximately 160 000 light years away in the Large Magellanic Cloud. This is a Milky Way neighbor. The strong gravitational force exerted on the black hole causes the star to distort its shape. The black hole's strong gravitational force not only distorts the star's shape but also affects its orbit. A team of astronomers was able to identify the existence of the black hole by looking at the subtle orbital effects. This makes it the first small black hole to be discovered outside of our galaxy. The Multi Unit Spectroscopic Explorer instrument (MUSE) at ESO's Very Large Telescope, Chile was used by the team to make this discovery. Credit: ESO/M. Kornmesser
Astronomers discovered a small black spot outside of the Milky Way using the European Southern Observatory's Very Large Telescope. This telescope is used to observe how the black hole affects the motion of stars in its vicinity. This detection method is new and it has revealed a black hole beyond our galaxy for the first time. This method may be the key to uncovering hidden black holes within the Milky Way or nearby galaxies. It could also help shed light on how these mysterious objects evolve.

The black hole, which was discovered in NGC 1850, is a cluster of thousands stars located approximately 160 000 light-years from the Large Magellanic Cloud. It is a neighbor galaxy to the Milky Way.

"This is similar to Sherlock Holmes pursuing a criminal gang, we are looking at each star in this cluster using a magnifying lens in one hand trying find evidence for the existence of black holes, but without actually seeing them," Sara Saracino, from the Astrophysics Research Institute of Liverpool John Moores University in the UK who led the research that was accepted for publication in Monthly Notices of the Royal Astronomical Society. The result here is only one of the wanted criminals. However, once you have located one, you can start to find others in other clusters.

The team found that the first "criminal", which was discovered by them, was approximately 11 times larger than our Sun. Astronomers discovered the black hole's gravitational effect on the five-solar mass star that orbits it. This was the key to their discovery.

Astronomers have previously detected such small black holes with "stellar mass" in other galaxies. This was done by observing the X-ray glow they emit as they swallow matter or the gravitational waves that are generated when black holes collide with each other or with neutron star.

Most stellar-mass black hole don't reveal their existence through X-rays and gravitational waves. Stefan Dreizler, who is a member of the team and is based at the University of Gottingen, Germany says that "the vast majority cannot be unveiled dynamically." "When they form a system together with a star, they will influence its motion in subtle but detectable ways. This allows us to find them using sophisticated instruments."

Saracino and her colleagues used a dynamical method to locate black holes. This could help astronomers unlock their mysteries. Mark Gieles, a study co-author from the University of Barcelona in Spain, said that every single discovery will help us understand the future of star clusters and black holes.

NGC 1850 is the first time that a black hole was detected in a young star cluster. The cluster is just 100 million years old. This is a blink of an astronomical eye. Their dynamical method could be used in star clusters similar to NGC 1850 to reveal more young black holes, and shed light on their evolution. Astronomers could compare them to older, larger black holes in other clusters to better understand how they grow. Charting the demographics and ages of black holes within star clusters can help us understand the origin of gravitational waves.

The team used data from the Multi Unit Spectroscopic Explorer, (MUSE), collected over two years at ESO's VLT in the Chilean Atacama Desert to carry out their search. MUSE allowed us to see very crowded areas like the innermost regions in stellar clusters and analyze the light from every star in the area. Sebastian Kamann, co-author and a long-time MUSE specialist based at Liverpool’s Astrophysics Research Institute, says that the net result includes information about thousands of stars, at most 10 times greater than any other instrument. The team was able to identify the strange star whose unusual motion indicated the existence of the black hole. The University of Warsaw's Optical Gravitational Lensing Experiment, and the NASA/ESA Hubble Space Telescope allowed them to determine the mass of the black holes and confirmed their findings.

The ESO's Extremely Large Telescope (Chile) will be operational in the latter part of this decade. It will enable astronomers to discover more black holes. Saracino says that the ELT will revolutionize this field. It will enable us to see stars at much higher distances and also to search for black holes in globular clusters.

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More information: S Saracino et al., A black hole was detected in the young massive LMC-cluster NGC 1850. Monthly Notices of the Royal Astronomical Society (2021). Information for the Journal: Monthly Notices from the Royal Astronomical Society, S Saracino et al., A black hole discovered in the young massive LMC-cluster NGC 1850 (2021). DOI: 10.1093/mnras/stab3159