Nestled in the heart of a tiny galaxy 54 million light years from Earth is a black hole so big it makes up 15% of the star cluster’s total mass.
The supermassive black hole, discovered at the centre of galaxy M60-UCD1, is said to have a mass equivalent to 21 million suns.
By comparison, the galaxy is 500 times smaller than the Milky Way, which itself has a black hole at its heart with a mass of just four million suns.
The galaxy, M60-UCD1, is the smallest ever found to have such a light-consuming gravitational monster at its core, they say.
Anil Seth of the University of Utah in Salt Lake City and his colleagues report the findings on 17 September in Nature. The team became intrigued by the ultra-compact dwarf galaxy M60-UCD1, some 16.6 million parsecs (54 million light years) from Earth, in part because its X-ray emissions suggested that it might house a black hole.
Images taken with the Hubble Space Telescope showed that the galaxy harboured a high concentration of mass at its centre, but the team had no idea how heavy the putative black hole might be.
The dwarf galaxy orbits M60, one of the largest galaxies near the Milky Way, at a distance of only about 22,000 light-years from the larger galaxy’s center, “closer than the sun is to the center of the Milky Way,” Seth said.
The scientists calculated the size of the supermassive black hole that may lurk inside M60-UCD1 by analyzing the motions of the stars in that galaxy, which helped the researchers deduce the amount of mass needed to exert the gravitational field seen pulling on those stars. For instance, the stars at the center of M60-UCD1 zip at speeds of about 230,000 mph (370,000 km/h), much faster than stars would be expected to move in the absence of such a black hole.
The supermassive black hole at the core of the Milky Way has a mass of about 4 million suns, taking up less than 0.01 percent of the galaxy’s estimated total mass, which is about 50 billion suns. In comparison, the supermassive black hole that may lie in the core of M60-UCD1 appears five times larger than the one in the Milky Way, and also seems to make up about 15 percent of the dwarf galaxy’s mass, which is about 140 million suns.
“That is pretty amazing, given that the Milky Way is 500 times larger and more than 1,000 times heavier than the dwarf galaxy M60-UCD1,”
The dwarf galaxy was probable much bigger originally, with a more usual black hole to galaxy mass ratio, the astronomers suggest, but a collision with a much bigger galaxy likely stripped off much of its outermost parts. That collision could have happened at least 10 billion years ago, they say.
Eventually, M60-UCD1 “may merge with the center of M60, which has a monster black hole in it, with 4.5 billion solar masses — more than 1,000 times bigger than the supermassive black hole in our galaxy,” Seth said in a statement. “When that happens, the black hole we found in M60-UCD1 will merge with that monster black hole.”
“This discovery could actually double the number of black holes in the universe,” said Anil Seth. “There are lots of ultra compact galaxies like this one, and it’s possible that many of them have black holes as well.”
It’s going to take more than one dwarf to prove that, but any opportunity to study black holes is a good one. “They’re part of the origin story of us, and our universe,” Seth said. “Every galaxy like ours has one of these, and we know that they affect how galaxies evolve and how stars form.” But we still don’t understand why black holes end up where they do. “If you want to understand how we got here, figuring out the role that black holes play is an important part of that,” Seth said.