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Oxymoronic Black Hole Provides Clues to Growth

For Release: August 11, 2015

NASA

RGG 118
Credit: X-ray: NASA/CXC/Univ of Michigan/V.F.Baldassare, et al; Optical: SDSS; Illustration: NASA/CXC/M.Weiss
Press Image and Caption

Astronomers using NASA's Chandra X-ray Observatory and the 6.5-meter Clay Telescope in Chile have identified the smallest supermassive black hole ever detected in the center of a galaxy. This oxymoronic object could provide clues to how larger black holes formed along with their host galaxies 13 billion years or more in the past.

Astronomers estimate this supermassive black hole is about 50,000 times the mass of the sun. This is less than half the mass of the previous smallest black hole at the center of a galaxy.

"It might sound contradictory, but finding such a small, large black hole is very important," said Vivienne Baldassare of the University of Michigan in Ann Arbor, first author of a paper on these results published in The Astrophysical Journal Letters. "We can use observations of the lightest supermassive black holes to better understand how black holes of different sizes grow."

The tiny heavyweight black hole is in the center of a dwarf disk galaxy, called RGG 118, located about 340 million light years from Earth, and was originally discovered using the Sloan Digital Sky Survey.

Researchers estimated the mass of the black hole by studying the motion of cool gas near the center of the galaxy using visible light data from the Clay Telescope. They used the Chandra data to figure out the X-ray brightness of hot gas swirling toward the black hole. They found the outward push of radiation pressure of this hot gas is about 1 percent of the black hole’s inward pull of gravity, matching the properties of other supermassive black holes.

Previously, scientists had noted a relationship between the mass of supermassive black holes and the range of velocities of stars in the center of their host galaxy. This relationship also holds for RGG 118 and its black hole.

"We found this little supermassive black hole behaves very much like its bigger, and in some cases much bigger, cousins," said co-author Amy Reines of the University of Michigan. "This tells us black holes grow in a similar way no matter what their size."

The black hole in RGG 118 is nearly 100 times less massive than the supermassive black hole found in the center of the Milky Way. It’s also about 200,000 times less massive than the heaviest black holes found in the centers of other galaxies.

Astronomers are trying to understand the formation of billion-solar-mass black holes from less than a billion years after the big bang, but many are undetectable with current technology. The black hole in RGG 118 gives astronomers an opportunity to study a nearby small supermassive black hole.

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