These black hole couples formed when two galaxies collided and merged with each other, forcing their supermassive black holes close together.
The black hole pairs were uncovered by combining data from a suite of different observatories including NASA's Chandra X-ray Observatory, the Wide-Field Infrared Sky Explorer Survey (WISE), and the ground-based Large Binocular Telescope in Arizona, researchers said.
"Astronomers find single supermassive black holes all over the universe," said Shobita Satyapal, from George Mason University in the US.
"But even though we've predicted they grow rapidly when they are interacting, growing dual supermassive black holes have been difficult to find," said Satyapal.
Researchers used optical data from the Sloan Digital Sky Survey (SDSS) to identify galaxies where it appeared that a merger between two smaller galaxies was underway.
From this set, they selected objects where the separation between the centers of the two galaxies in the SDSS data is less than 30,000 light years, and the infrared colors from WISE data match those predicted for a rapidly growing supermassive black hole.
Seven merging systems containing at least one supermassive black hole were found with this technique.
Since strong X-ray emission is a hallmark of growing supermassive black holes, researchers then observed these systems with Chandra.
Closely-separated pairs of X-ray sources were found in five systems, providing compelling evidence that they contain two growing (or feeding) supermassive black holes.
Both the X-ray data from Chandra and the infrared observations suggest that the supermassive black holes are buried in large amounts of dust and gas.
"X-rays and infrared radiation are able to penetrate the obscuring clouds of gas and dust surrounding these black hole pairs, and Chandra's sharp vision is needed to separate them," said Ellison.
One member of this black hole pair is particularly powerful, having the highest X-ray luminosity in a black hole pair observed by Chandra to date.
The research has implications for the burgeoning field of gravitational wave astrophysics.
While scientists using the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the VIRGO interferometer have detected the signals of merging black holes, these black holes have been of the smaller variety weighing between about eight and 36 times the mass of the Sun.
The merging black holes in the centres of galaxies are much larger. When these supermassive black holes draw even closer together, they should start producing gravitational waves.
The eventual merger of the dual supermassive black holes in hundreds of millions of years would forge an even bigger black hole.
This process would produce an astonishing amount of energy when some of the mass is converted into gravitational waves.
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