Dwarf galaxy suggests black holes may be more common than previously thought

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Artists impression of M60-UCD1 – a dwarf galaxy with a supermassive black hole at its center (Image: NASA/ESA)

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Astronomers from the University of Utah have discovered a dwarf galaxy that is the smallest ever recorded with a supermassive black hole at its center. The galaxy, M60-UCD1, which is located around 54 million light years from our solar system near the M60 galaxy, has been found to contain a black hole with a mass equivalent to 21 million times that of our own sun and whose presence may suggest that such enormous black holes could be more common than previously thought.

"It is the smallest and lightest object that we know of that has a supermassive black hole," said Anil Seth, lead author of the dwarf galaxy study. "It’s also one of the most black hole-dominated galaxies known."

The researchers claim that their discovery, which was made using the Gemini North telescope on Hawaii’s Mauna Kea and images captured by the Hubble Space Telescope, indicates that a large number of other ultra-compact galaxies may also harbor supermassive black holes. Furthermore, they also believe these diminutive galaxies could be all that remains of larger galaxies that have been ripped asunder during 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, an assistant professor of physics and astronomy at the University of Utah. "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."

Our own Milky Way contains a black hole at its center, however its mass is small compared to M60-UCD1's black hole, at "only" 4 million times the mass of our sun. And, at less than 0.01 percent of the Milky Way’s total mass, it is positively miniscule in comparison to the supermassive black hole in M60-UCD1, which weighs in at a gigantic 15 percent of the total mass of that galaxy.

"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," said Seth. "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. That was maybe as much as 10 billion years ago. We don’t know."

Dwarf galaxies are generally classified as being less than a few hundred light years across – around 1,700 trillion miles (2,735 trillion km) wide – compared with our Milky Way’s 100,000-light-year diameter. M60-UCD1 fits into that category, and whilst most dwarf galaxies exist at relatively large distances from other galaxies, this one is located only 22,000 light years from the center of galaxy M60; much closer to that galactic center than our sun is to the center of our own Milky Way.

"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," Said Seth. "When that happens, the black hole we found in M60-UCD1 will merge with that monster black hole."

To estimate the mass of the M60-UCD1’s supermassive black hole, the astronomers used the 8 m (26 ft) diameter Gemini North telescope – the same instrument recently used to take a snap of exoplanet Beta Pictoris b – to gauge the velocity and movement of stellar objects orbiting it. The researchers found that the stars located in the center of M60-UCD1 travel at around 230,000 mph (370,000 km/h), which is faster than stars are ordinarily expected to move, indicating the influence of a black hole. As a result of their observations, the researchers demonstrated that the galaxy is more massive than the amount of starlight it emits would indicate.

Though the theory expounded by the researchers may also possibly indicate that M60-UCD1 is simply made up of a large amount of massive, dim stars, and not as a result of a supermassive black hole, the team believe that its observations confirmed that the mass was concentrated in the galaxy’s center, and this indicated a supermassive black hole. The astronomers also relied on previous research that showed M60-UCD1 was an X-ray source and that gas was being drawn into the center at a rate that indicated similarities to other supermassive black holes in much bigger galaxies.

The researchers recently published their results in the journal Nature.

The video below shows an animation of the theorized formation of dwarf galaxy M60-UCD1 by astronomer Holger Baumgardt of the University of Queensland.

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