Astronomers noticed something curious about the center of a galaxy 600 million light-years away. A central blast of radiation, a quasar, seemed to have an engine of bumper-sized proportions. Not one, but two black holes circling each other.
The presence of a quasar may be a clue to the galaxy's past. Or more precisely, the galaxies' pasts. When two galaxies collide, their central black holes start to whirl around each other, caught in a gravitational dance. The power created from both the merger and the orbiting black holes is tremendous and plentiful to fuel a brilliant quasar.
Astronomers used NASA's Hubble Space Telescope to find this galaxy, which has the catchy name of Markarian 231 (Mrk 231). It has the proud title of being the closest galaxy to planet Earth that contains a quasar.
A quasar is an extremely luminous object: they can be 100 times brighter than the Milky Way. They are formed around supermassive black holes. A supermassive black hole sucks matter into its clutches, but before the hot dust and matter fall into the black hole, they swirl around it on a disk, known as an accretion disk. The friction caused by the fast matter spinning around the black hole heats up the disk. Eventually, the heat is too much to bear, and a jet of radiation shoots out: a quasar.
While probing this particular quasar, the light spectrum of the galaxy revealed some surprising results in the ultraviolet range. The scientists discovered that there were "extreme and surprising properties." You can see the results in The Astrophysical Journal.
What they would normally expect is a bright reading in the ultraviolet spectrum, corresponding to a single black hole at the center of the galaxy. However, this characteristic glow was distinctly lacking. There was a dip in intensity over the center – like a donut-shaped hole – where it should have been bright.
The dip, the scientists propose, could be explained by two black holes orbiting each other, carving out a hole in the center of the disk in the process.
“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,” said Youjun Lu from the National Astronomical Observatories of China, in a statement.
“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,” added co-investigator Xinyu Dai, of the University of Oklahoma.