The vast majority of black holes we have observed are in two very different classes. Either they are stellar size, weighing up to a few tens of times the mass of our Sun; or they are supermassive, with a weight of millions, if not billions of times our star. The intermediate-mass black holes remain an elusive class, sitting right in the middle.
A few members of this group have been discovered, and the latest one is in the nearby Andromeda galaxy. It was discovered in the collection of stars known as B023-G078 and weighs about 100,000 times the mass of the Sun. A similarly sized black hole was recently discovered in a dwarf galaxy.
B023-G078 was believed to be a globular cluster orbiting the Andromeda Galaxy, but this discovery has shifted that view. In their work, published in The Astrophysical Journal, researchers suggest that it might instead be a stripped core of a galaxy that has been ripped apart by Andromeda. Its total mass is over 6 million times our Sun.
"I knew that the B023-G078 object was one of the most massive objects in Andromeda and thought it could be a candidate for a stripped nucleus. But we needed data to prove it. We'd been applying to various telescopes to get more observations for many, many years and my proposals always failed," senior author Anil Seth, associate professor of astronomy at the University of Utah, said in a statement.
"When we discovered a supermassive black hole within a stripped nucleus in 2014, the Gemini Observatory gave us the chance to explore the idea."
While the collection of stars looks enough like a globular cluster, there have been some suspicions that B023-G078 was hiding many things about its true nature. Researchers studied the mass distribution and chemical composition and found that it varied – globular clusters tend to form at the same and be uniform, and this was not the case.
They also studied the motion of the stars in this stripped core, and found that they were moving in a peculiar way. They were orbiting something quite massive, located in a small area of space.
"The stellar velocities we are getting gives us direct evidence that there's some kind of dark mass right at the center," lead author Dr Renuka Pechetti of Liverpool John Moores University.
"It's very hard for globular clusters to form big black holes. But if it's in a stripped nucleus, then there must already be a black hole present, left as a remnant from the smaller galaxy that fell into the bigger one."
Finding this object is a two-in-one astrophysics deal. Stripped cores are rare and provide insights into galaxy formation because all big galaxies grow by cannibalizing smaller ones, and intermediate-mass black holes are also rare and might tell us how smaller galaxies evolve – and maybe how supermassive black holes got so big.