Surprise! Giant Black Hole Discovered in Dwarf Galaxy

Astronomers have just discovered the smallest known galaxy that harbors a huge, supermassive black hole at its core.

The relatively nearby dwarf galaxy is thought to house a supermassive black hole at its heart equal in mass to about 21 million suns. The discovery suggests that supermassive black holes may be far more common than previously thought.

A supermassive black hole, millions to billions of times the mass of the sun, lies at the heart of nearly every large galaxy like the Milky Way. Scientists were uncertain whether dwarf galaxies, roughly one-fiftieth the brightness of the Milky Way, might also harbor supermassive black holes. [Watch a video about the new dwarf galaxy finding]

Image: Black hole
This is an illustration of the supermassive black hole located in the middle of the very dense galaxy M60-UCD1. It weighs as much as 21 million times the mass of our sun. Lying about 50 million light-years away, M60-UCD1 is a tiny galaxy with a diameter of 300 light-years — just one-500th of the diameter of the Milky Way. NASA, ESA, D. Coe, G. Bacon / STScI

To find out, University of Utah astronomer Anil Seth and his colleagues investigated M60-UCD1, an ultra-compact dwarf galaxy, using the Gemini North 8-meter optical-and-infrared telescope on Hawaii's Mauna Kea volcano and NASA's Hubble Space Telescope. M60-UCD1 lies about 54 million light-years away from Earth. The dwarf galaxy orbits M60, one of the largest galaxies near the Milky Way.

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. The black hole appears to be five times larger than the one at the center of the Milky Way. "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," Seth said in a statement.

The researchers suggest that M60-UCD1 was once a very large galaxy, "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," Seth said.

Seth said M60-UCD1 and its black hole may eventually merge with M60, which has an even bigger black hole at its center.

The scientists detailed their findings in Thursday's issue of the journal Nature.

— Charles Q. Choi,

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