Photo: ESO / M. Grain knives
Using the Very Large Telescope (VLT) at the European Southern Observatory (ESO), astronomers have been able to locate a small black hole outside our Milky Way by looking at how it affects the motion of a star in its vicinity. It is the first time that this detection method has been used to detect a black hole outside our Milky Way. This method may be crucial for uncovering hidden black holes in the Milky Way and nearby galaxies, and could shed light on how these mysterious objects formed and evolved. The newly discovered black hole was hidden in NGC 1850, a group of thousands of stars about 160,000 light-years away in the Large Magellanic Cloud, a small, neighboring galaxy of the Milky Way.
“Much like Sherlock Holmes tracking down a criminal gang with their missteps, we look at every star in this constellation through a magnifying glass and try to find evidence of black holes without seeing them directly,” he says. The Astrophysical Research Institute at Liverpool John Moores University in the UK, who oversaw the research that has now been accepted for publication in the Monthly Notices of the Royal Astronomical Society. “The score shown here is just one of the wanted criminals, but if you find one, you’re on the right track to discovering more hiding in other star clusters.” The first ‘huddler’ the team tracked turned out to be 11 times the mass of our sun. Astronomers track it by the gravitational effect it exerts on a five-solar-mass star orbiting it. Previously, astronomers were able to detect such small black holes in other galaxies by detecting X-rays emitted by black holes when they swallow matter or by gravitational waves emitted by black holes when they collide with each other or with neutron stars.
However, most black holes in some stellar mass do not betray their existence by emitting X-rays or gravitational waves. “The vast majority can only be detected through dynamics,” says team member Stefan Dreitzler, from the University of Georg-August-Göttingen in Germany. “When they have an ordinary star as their companion, they affect its movement in a subtle but detectable way, so we can track them down with advanced tools.” Using the dynamic method used by Saracino and her team, astronomers have been able to detect and decipher many black holes. “Any discovery we make will be important to our understanding of star clusters and the black holes within them,” said co-author Mark Gillis of the University of Barcelona, Spain.
It’s the first black hole to be found in a young star cluster (NGC 1850 is only about 100 million years old – the blink of an eye by astronomical standards). By applying the dynamic method to groups of similar stars, astronomers can track down more young black holes and compare them to larger, more mature black holes in older star groups. In this way, more ideas can be gained about how these objects can “grow” by feeding on stars or merging with other black holes. In addition, mapping the demographics of black holes in star clusters would improve our understanding of the origin of the sources of gravitational waves. In their research, the astronomers used data collected over two years using the Multi-Unit Spectral Explorer (MUSE), along with ESO’s VLT, located in the Atacama Desert in Chile. MUSE allowed us to observe densely populated regions, such as the interior of star clusters, and analyze the light from every star in the vicinity. Co-author Sebastian Kaman, a MUSE expert at the Liverpool Institute for Astrophysical Research, said the instrument can gather information on thousands of stars in one hit, at least ten times more than any other instrument. This made it possible to discover a star whose strange motion betrayed the existence of the black hole. Data from the University of Warsaw’s Gravitational Optical Lens Experiment and from the NASA/ESA Hubble Space Telescope enabled astronomers to confirm their findings and measure the mass of the black hole.
With ESO’s Very Large Telescope in Chile, due to become operational later this decade, astronomers will be able to discover even more hidden black holes. “ELT is sure to revolutionize the field,” Saracino says. This will allow us to observe noticeably faint stars in the same field of view, as well as search for black holes in globular clusters at much greater distances.
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