Astronomers say the discovery of a giant black hole at the heart of a tiny dwarf galaxy -- an unexpected find in a galaxy so compact -- suggests black holes might be more numerous than thought.
The galaxy, M60-UCD1, is the smallest ever found to have such a light-consuming gravitational monster at its core, they say.
"It is the smallest and lightest object that we know of that has a supermassive black hole," says Anil Seth of the University of Utah, lead author of a study by an international team published in Nature.
The discovery suggests similar ultracompact dwarf galaxies could possess supermassive black holes, the astronomers say, with the most likely explanation for the unexpected finding being that such dwarf galaxies may be the remnants of much larger examples torn apart in collisions with other galaxies.
"We don't know of any other way you could make a black hole so big in an object this small," says Seth, a professor of astronomy and physics. "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."
M60-UCD1 is much smaller than our own Milky Way galaxy, with just 0.2 percent the mass, yet its black hole makes up 15 percent of its mass, compared with the .01 percent represented by our own black hole.
The dwarf galaxy's black hole is nearly five times as big as the one in the Milky Way, Seth says.
"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.
M60-UCD1, about 54 million light years distant from Earth, is the most massive ultracompact dwarf galaxy -- the densest star system found in the universe -- discovered to date, with a total of 140 million solar masses, the researchers report.
The discovery of the dwarf galaxy's black hole, and the likelihood of there being more within similar galaxies, is strong evidence for more black holes existing than has been thought up to know, they say.
"This discovery could actually double the number of black holes in the universe," says Seth.
