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Supermassive black hole found in smallest galaxy

WASHINGTON, Sept. 18 — An international team of researchers has found a supermassive black hole at the heart of an ultracompact dwarf galaxy, making this tiny galaxy the smallest ever found to host such a massive light-sucking object.

Ultracompact dwarf galaxies are among the densest star systems in the universe. Their composition, however, has been less clear, although supermassive black holes — those with the mass of at least one million stars like our sun — are believed to lie at the center of nearly all large galaxies.

The new findings, published Wednesday in the British journal Nature, suggested that many other ultracompact dwarf galaxies may also host supermassive black holes.

"It is the smallest and lightest object that we know of that has a supermassive black hole," lead author Anil Seth, assistant professor of the University of Utah said in a statement.

"It's also one of the most black hole-dominated galaxies known. "

The astronomers used the Gemini North 8-meter optical-and- infrared telescope on Hawaii's Mauna Kea and photos taken by the Hubble Space Telescope to discover that a small galaxy named M60- UCD1, roughly 54 million light years from Earth, contains more mass than would be expected by the amount of starlight it emits.

Further analysis revealed that mass was concentrated in the galaxy's center, indicating the presence of a supermassive black hole.

They calculated that the supermassive black hole at the center of M60-UCD1 has a mass of 21 million suns, making up a stunning 15 percent of the small galaxy's total mass of 140 million suns.

By comparison, the supermassive black hole at the center of our Milky Way galaxy has only a mass of 4 million suns, or one-fifth that of M60-UCD1, and it is less than 0.01 percent of the galaxy's total mass, estimated at some 50 billion solar masses.

"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.

He said M60-UCD1 might be the stripped center or nucleus of what once was a much larger galaxy, possibly about 10 billion solar masses, or about one-fifth the mass of the Milky Way.

"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," he said. "That was maybe as much as 10 billion years ago."

Seth said ultracompact dwarf galaxy M60-UCD1 may be doomed, although he cannot say when because the dwarf galaxy's orbit around M60 isn't known.

"Eventually, this thing 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. When that happens, the black hole we found in M60-UCD1 will merge with that monster black hole."

The study, conducted by Seth and 13 other astronomers, was funded by the National Science Foundation (NSF) in the U.S., the German Research Foundation and the Gemini Observatory partnership, which includes the NSF and scientific agencies in Canada, Chile, Australia, Brazil and Argentina. (PNA/Xinhua)


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