By Managing editor
updated 6/13/2011 2:13:30 PM ET 2011-06-13T18:13:30

A galaxy already known to have one ginormous black hole at its core is actually home to two of these cosmic giants, a new study reveals.

Astronomers discovered the second monster black hole at the center of the galaxy Markarian 739, which is about 425 million light-years from Earth, toward the constellation Leo. Its presence was revealed in observations by NASA's Swift satellite and the Chandra X-Ray Observatory.

The two black holes are separated by about 11,000 light-years, which is about one-third the distance between our solar system and the center of the Milky Way. One light-year is about 6 trillion miles.

Supermassive black hole twins
Both black holes are intensely active and classified as "supermassive," that is, they can each have a mindboggling mass equivalent to millions — or even billions — of stars like our sun.  Stellar black holes, formed by the collapse of massive stars, typically have up to 10 or 20 times the mass of the sun.  

"At the hearts of most large galaxies, including our own Milky Way, lies a supermassive black hole weighing millions of times the sun's mass," the study's lead author, Michael Koss,  a researcher at NASA's Goddard Space Flight Center and the University of Maryland in College Park, said in a statement. "Some of them radiate billions of times as much energy as the sun."

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While supermassive black holes are relatively common at the core of galaxies, not all are radiating energy to be what astronomers call "active galactic nuclei" (AGN). So it is rare to find one active monster black hole, let alone two, in the same galaxy, researchers said.

When galaxies collide
Astronomers suspect that the active binary supermassive black hole setup can occur when galaxies collide.

"If two galaxies collide and each possesses a supermassive black hole, there should be times when both black holes switch on as AGN," said study co-author Richard Mushotzky, also of the University of Maryland in College Park.

The astronomers discovered the dual black hole heart of Markarian 739 by using NASA's Swift satellite's Burst Alert Telescope, which maps intense sources of X-ray emissions in the sky, to seek out potential active galactic nuclei. Researchers can then zoom in on potential candidates with the Chandra X-Ray Observatory.

SDSS
When viewed in visible light, Markarian 739 resembles a smiling face. It is home to a rare set of two active, supermassive black holes about 425 million light-years away and may have once been two galaxies that are now merging.

Markarian 739's second black hole was invisible in the ultraviolet, visible and radio ranges of the light spectrum, so it remained hidden until this new study, researchers said. The findings will be detailed in an upcoming edition of The Astrophysical Journal Letters.

The two giant black holes at the center of Markarian 739, which is also known by the name NGC 3758, are not the first monster black hole twins seen by astronomers, and they're not the closest either.  Both records are held by the galaxy NGC 6240, a galaxy that is about 330 million light-years from Earth.

You can follow Space.com managing editor Tariq Malik on Twitter @tariqjmalik. Follow Space.com for the latest in space science and exploration news on Twitter @Spacedotcomand on Facebook.

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