Finally found: Binary black hole systems (pictured) should be common, but have been hard to find. Now researchers think they’ve discovered a pair that orbit around each other once every 100 years.
Credit: P. Marenfeld, NOAO
BRISBANE: A galaxy has been detected that appears to have not one, but two supermassive black holes at its core, which are orbiting one another like binary stars.
Although astronomers have long suspected that binary black holes must exist, this is the first time a likely candidate has been spotted, according to a report in the British journal Nature today.
Difficult to find
"There ought to be lots of them, but they have been difficult to identify," said study co-author Todd Boroson, an astronomer at the National Optical Astronomy Observatory (NOAO) in Tucson, Arizona. "Perhaps from studying this one unambiguous case, we can learn how to find more."
The binary black holes are five billion light years away from Earth, and form a quasar.
Quasars appear like stars from Earth, but are in fact the extremely bright nuclei of distant galaxies, formed by material at great heat and pressure being sucked towards a black hole.
Boroson and co-worker Tod Lauer were studying quasars found by the Sloan Digital Sky Survey when they noticed that the information from one quasar was inconsistent with the rest of the sample.
This quasar didn't show all the characteristics of any single type of quasar, Boroson said, and, even more strangely, it had two broad emission lines instead of just one.
These broad emission lines are luminous spectral beams emitted by material accelerating as it falls into a black hole, so the two beams indicated that there were two supermassive black holes in the quasar.
Eventual merger
The black holes appear to be 20 million and one billion times the mass of our Sun, respectively, and seem to be orbiting one another once every 100 years or so. They are travelling at around 6,000 km/s and the distance that separates them is just one tenth of the distance between our Sun and the next nearest star.
Their angular momentum is currently preventing them from being dragged together, though they will eventually merge, Boroson said.
Most galaxies have a supermassive black hole at their centre, and many galaxies have collided and merged during the history of the universe, so binary black holes should be common, he added.
Another possible explanation is that the quasar in question might actually be two separate quasars that happen to be in the same line of sight from Earth, but Boroson said the probability of two such objects lining up exactly is very low and one of the objects still would not behave like other quasars.
John Dickey, an astronomer at the University of Tasmania in Hobart, Australia, said that, while it was hard to calculate the probability of two quasars lining up, the case for the binary black hole explanation was very convincing. "It's pretty strong evidence," he said.
Boroson and his colleagues hope to bolster that evidence by observing the quasar over the next few years, because the velocity of the broad emission lines should gradually change as the black holes orbit each other.
"We are also hoping to get an image of the object with the Hubble Space Telescope, to confirm that it doesn't look like two separate objects," said Boroson.
