A very dark dance is going on about 600 million light-years away from Earth. That's where the closest quasar to our planet lies, and scientists recently figured out that it is fueled by two black holes at its center.

Quasars lie at the core of some galaxies and are the brightest objects in the known universe. It has been known for decades that supermassive black holes power their extreme luminosity by ejecting all of the particles that don't fall into its depths outward at nearly the speed of light. However, until scientists going through archived Hubble Space Telescope observations noticed that a quasar known as Markarian 231 (Mrk 231) had certain "extreme and surprising properties," it was thought that each quasar had just one enormous black hole at its core.

Binary black holes are pairs of black holes that orbit one another. Scientists realized that such a duo danced at the core of quasar Mrk 231 when they looked at the data on the ultraviolet (UV) radiation that it emits. A quasar with a single black hole at its core would have a complete disk of dust glowing with UV rays, but Mrk 231's disk looks more like a donut.

"The structure of our universe, such as those giant galaxies and clusters of galaxies, grows by merging smaller systems into larger ones, and binary black holes are natural consequences of these mergers of galaxies," co-investigator Xinyu Dai of the University of Oklahoma explained in a statement.

This donut-shaped disk of dust is best explained by the existence of a pair of black holes. As the two black holes orbit around one another, they carve out the center of the disk.

The black hole closer to the center of the quasar is a black hole 150 million times as massive as our sun. Its companion is smaller, just four million times the mass of our sun. Still, neither is nearly the size of supermassive black holes, which are millions or even billions of times as massive as our sun. However, since binary black holes orbit around one another, they generate additional energy, like a pair of figure skaters twirling together.

"We are extremely excited about this finding because it not only shows the existence of a close binary black hole in Mrk 231, but also paves a new way to systematically search binary black holes via the nature of their ultraviolet light emission," Youjun Lu of the National Astronomical Observatories of China, Chinese Academy of Sciences, said in a statement.

Scientists expect that, in a few hundred thousand years, the two black holes of Mrk 231 will spiral into each other.

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