Monster Black Hole Twins Found Inside Galaxy's Belly

Viewed in visible light, Markarian 739 resembles a smiling face. It actually is a pair of merging galaxies which lies 425 million light-years away.
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. (Image credit: SDSS)

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 (10 trillion kilometers). [Photos: Black Holes of the Universe]

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," said the study's lead author Michael Koss,  a researcher at NASA's Goddard Space Flight Center and the University of Maryland in College Park, in a statement. "Some of them radiate billions of times as much energy as the sun."

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.

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.

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Tariq Malik

Tariq is the Editor-in-Chief of and joined the team in 2001, first as an intern and staff writer, and later as an editor. He covers human spaceflight, exploration and space science, as well as skywatching and entertainment. He became's Managing Editor in 2009 and Editor-in-Chief in 2019. Before joining, Tariq was a staff reporter for The Los Angeles Times covering education and city beats in La Habra, Fullerton and Huntington Beach. In October 2022, Tariq received the Harry Kolcum Award for excellence in space reporting from the National Space Club Florida Committee. He is also an Eagle Scout (yes, he has the Space Exploration merit badge) and went to Space Camp four times as a kid and a fifth time as an adult. He has journalism degrees from the University of Southern California and New York University. You can find Tariq at and as the co-host to the This Week In Space podcast with space historian Rod Pyle on the TWiT network. To see his latest project, you can follow Tariq on Twitter @tariqjmalik.