Astronomers from the University of Utah recently found a supermassive black hole in a very small dwarf galaxy, M60-UCD1, one of the smallest galaxies currently known.
This discovery could mean that other similar dwarf galaxies, which are dense, could also have large black holes, meaning that black holes, in general, are more common than originally thought.
Astronomers also now believe that these galaxies were once part of other larger galaxies that separated after collisions with yet other galaxies.
"We don't know of any other way you could make a black hole so big in an object this small," says Anil Seth, an assistant professor of physics and astronomy at the University of Utah and lead author of the study. "There are a lot of similar ultracompact dwarf galaxies, and together they may contain as many supermassive black holes as there are at the centers of normal galaxies."
Black holes are collapsed stars that have so much gravity that nothing can escape from them, not even light. Black holes emit radiation, like X-rays, which allows telescopes, both on Earth and in space, to identify them. In this case, both the Gemini North telescope in Hawaii and the Hubble Space telescope picked up these x-ray emissions in M60-UCD1.
This newly discovered supermassive black hole in M60-UCD1 is huge: its center is about the mass of 21 million of our suns. In comparison, the black hole at the center of the Milky Way only has a mass of about 4 million suns. Considering how small M60-UCD1 is, at just a few hundred light years across, the black hole accounts for nearly 15 percent of the entire galaxy's mass. By comparison, the Milky Way's black hole only accounts for 0.01 percent of our galaxy's mass.
M60-UCD1 is a special kind of dwarf galaxy: it is also considered supercompact. This means it has some of the densest population of stars in the Universe. These dense galaxies are often a topic of debate for scientists. Are they huge clusters of stars all born at the same time or are they remnants of galaxy collisions? Seth's team believes it's the latter.
"We believe this once was a very big galaxy with maybe 10 billion stars in it, but then it passed very close to the center of an even larger galaxy, M60, and in that process all the stars and dark matter in the outer part of the galaxy got torn away and became part of M60," says Seth. "That was maybe as much as 10 billion years ago."
M60-UCD1 is about 54 million light years from Earth.