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NGC 4395's pint-size black hole

This galaxy's central black hole is only 1 percent the mass astronomers expected to find.
Black Hole Jet
The plasma in a galactic jet is propelled outward by a galaxy's central, supermassive black hole.
NASA / GSFC
February 21, 2005
Astronomers using an innovative new technique for measuring the distribution of matter in distant galaxies have found an unusual black hole — the smallest of its kind discovered so far.

The black hole, discovered in the center of a galaxy known as NGC 4395, has a mass less than a million times that of the Sun. That might sound like a lot, but it's many times smaller than the normal "supermassive" black holes astronomers find in galaxies' centers.

"Typically, in the centers of galaxies, we'll find black holes that are a million to a billion solar masses," said astronomer Bradley Peterson of Ohio State University. "This one is less than a million — it's somewhere around 300,000 to 400,000 solar masses."

Peterson, who had been collaborating with colleagues in the United States and Israel, presented his results on Saturday at the American Association for the Advancement of Science meeting in Washington, D.C.

It's not just the black hole that's unusual — its host galaxy is an oddball as well, Peterson explained. While most galaxies are disk-shaped with a bulge in the middle, NGC 4395 is essentially flat; it has no central bulge. And this unusual shape may be linked to the size of the black hole: Peterson believes the black hole has already "eaten" all the stars in the center of the galaxy. This would explain why the black hole doesn't seem to be growing as well as the galaxy's odd shape.

"The remarkable thing about the galaxy that we studied is that it does not have this central bulge," Peterson said. "It just seems to be a pure disk. And despite the fact that it's a pure disk, it does have a fairly massive black hole — although it's very small by the standards of normal galactic black holes."

The black hole at the center of NGC 4395, located some 14 million light-years from Earth, could be thought of as "the runt of the litter," Peterson said. "It's the small black hole that never got enough to eat."

The astronomers used a relatively new technique called "reverberation mapping" to determine the black hole's properties. The method is similar to the Doppler radar meteorologists use to track storms. They examined light emitted from the galaxy's core and timed how long those signals took to reach gas orbiting the center. From this, they worked out the speed of the gas and, in turn, the size of the black hole.

Over the past decade or so, astronomers have found that nearly all normal galaxies, including our own Milky Way, have supermassive black holes at their centers. But it's still not clear whether the galaxies evolved first and the black holes later, or vice-versa, or whether they evolved together.

"We have an imperfect understanding of galaxies without classical bulges, and do not yet understnd their relation with central black hole mass," said astronomer Tim de Zeeuw of Leiden University in the Netherlands, commenting on the finding by e-mail. The case of NGC 4395's unusually small black hole "should help us begin to understand the presence and role of these black holes precisely in galaxies without classical bulges."
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