Astronomers first hypothesized that our galaxy contained a supermassive black hole at its center several decades ago. Once it had been confirmed, they turned to other nearby galaxies, finding that this was a common phenomenon–possibly even universal among large galaxies. Now astronomers have used the Hubble Space Telescope to uncover a supermassive black hole in a very unlikely place–at the center of one of the smallest galaxies ever discovered.
The object is known by the name M60-UCD1, indicating it is closely associated with the much larger M60 galaxy about 55 million light years away from Earth. At the center of this dwarf galaxy is a black hole five times more massive than the one at the center of the Milky Way, but the galaxy itself consists of only 140 million stars. The Milky Way, by comparison, has roughly 300 billion stars. The stars in M60-UCD1 are also packed very close together in a space only 300 light years across. That’s 0.2% as wide as the Milky Way. If you were to look up at the night sky in M60-UCD1, it would be a blazing sea of more than one million points of light.
So clearly the existence of a supermassive black hole is unusual in such a tiny galaxy. It is thought that these objects become so huge through the accretion of matter and merging with other black holes. That should only happen in a larger galaxy, though. So how did the M60-UCD1 dwarf galaxy come to exist?
Astronomers have a few hypotheses to test, but the most likely possibility is that M60-UCD1 was once a much larger galaxy of 10 billion stars. Then it got too close to the much larger M60 galaxy, which siphoned off most of its mass. M60 is one of the largest galaxies ever discovered with a black hole at its center about 1000 times more massive than the one in our galaxy.
If this is indeed how M60-UCD1 came to be, it would have interesting consequences for the study of dwarf galaxies. It’s possible these objects are not all pockets of stars formed on their own in tiny islands, but the remnants of much larger galaxies that lost their mass to a heavier neighbor.