This artist’s impression shows a compact black hole 11 times as massive as the Sun and the five-solar-mass star orbiting it. The two objects are located in NGC 1850, a cluster of thousands of stars roughly 160 000 light-years away in the Large Magellanic Cloud, a Milky Way neighbour. The distortion of the star’s shape is due to the strong gravitational force exerted by the black hole. Not only does the black hole’s gravitational force distort the shape of the star, but it also influences its orbit. By looking at these subtle orbital effects, a team of astronomers were able to infer the presence of the black hole, making it the first small black hole outside of our galaxy to be found this way. For this discovery, the team used the Multi Unit Spectroscopic Explorer (MUSE) instrument at ESO’s Very Large Telescope in Chile. Credit:ESO/M. Kornmesser

The "stellar-mass" black hole was discovered 160 000 light years away in the star cluster NGC 1850 in the Large Magellanic Cloud, a neighbouring galaxy to the Milky Way. Understanding black holes in star clusters is important to understanding the origin of gravitational waves as the mergers of black holes are thought to be more common in the dense environments of star clusters.

“Since we found the black hole in a star cluster, we have a much better idea how old the black hole is - 100 million years - than we would otherwise” explains Dr. Usher. "This is the first time a black hole has been found in such a young, astronomically speaking, cluster which allows us to start to understand how many and what sorts of black holes form in stellar clusters. By comparing the black holes in young star clusters with those in much older (ten billion years or more) star clusters, we will be able understand how black holes grow in mass in these systems.”

Similar black holes have previously been found either from the intense radiation emitted when matter falls into them or through gravitational waves that result when they merge with another black hole or a neutron star. But for most of their lives black holes are not eating or merging and dynamical techniques, like the one used here, are needed to locate them. 

To find the black hole, the researchers used the European Southern Observatory’s Very Large Telescope in Chile to measure how fast thousands of stars in the star cluster move towards us or away from us over the course of two years. The companion star to the black hole stood out as changing speed by 300 km per second every five days. Careful mathematical modelling of the light and motion of the companion star, including observations with the Hubble Space Telescope, revealed that the visible star has a mass of five times the Sun and the black hole a mass of eleven times that of the Sun. Dr. Saracino and the team aim to apply the technique to more star clusters to discover additional black holes.

The research was published online today in the Monthly Notices of the Royal Astronomical Society.