Supermassive black holes are located at the center of almost every galaxy and they have the mass of millions, if not billions, of Suns. Astronomers believe they form through the merging of intermediate-mass black holes, but these have never been detected in the Milky Way – until now.
In a paper published in Nature, researchers from Harvard and University of Queensland have announced the discovery of an intermediate-mass black hole in the stellar cluster 47 Tucanae. The object weighs 2,200 times the mass of the Sun and is located 13,000 light-years from Earth.
Before this discovery, it seemed that black holes came in two varieties: a small stellar-size, from a few to tens of times the mass of the Sun, and supermassive that range from millions to billions of times our star. Supermassive black holes are at the center of almost every galaxy and their evolution is believed to be linked to the intermediate-mass ones.
"We want to find intermediate-mass black holes because they are the missing link between stellar-mass and supermassive black holes," lead author Bülent Kiziltan, from the Harvard-Smithsonian Center for Astrophysics (CfA), said in a statement. "They may be the primordial seeds that grew into the monsters we see in the centers of galaxies today."
A candidate intermediate-mass black hole was observed in 2012 in a galaxy 300 million light-years away, but scientists are still working on its properties.
Discovering the monster lurking in 47 Tucanae wasn’t easy. Black holes, as the name suggest, can’t be seen directly. Astronomers wait for one to be feasting on stars or gas in order to catch the powerful light emitted in those events.
Unfortunately, 47 Tucanae is devoid of gas, so the team focused on more indirect methods. They looked at how stars and pulsars in the cluster moved. This work required sophisticated and detailed analysis, but the data doesn’t lie. The trajectories and speeds of the stars clearly show that a large black hole is hiding at the center of the cluster.
This discovery tells us that many more of these intermediate-mass black holes might be lurking in our galaxy undetected. And it also tells us that while they were important in the early universe, their part is still not over.