Researchers have uncovered a rarely seen gathering of three supermassive black holes in a pair of merging galaxies more than 4 billion light-years away.
The two elliptical galaxies are overlapping sufficiently that a supermassive black hole in one but 24,000 light-years from a close pair in the center of the second galaxy. For comparison, that's less than half way to the center of the Milky Way.
Of special interest is the close pair, which orbit each other at a distance of only about 450 light-years. Einstein's theory of general relativity predicts that their orbital minuet should literally make waves in the fabric of space-time, a phenomenon that researchers are trying hard to detect.
Moreover, the motions of two or three co-orbiting supermassive black holes is akin to that of a Mixmaster and are thought to affect the structure of their host galaxy by hurling dust, gas, and stars away from the galactic center, in effect hollowing it out.
Eventually, theory suggests, supermassive black holes in close binaries merge, setting off yet more ripples in space-time and leading to a more massive supermassive black hole at their host galaxy's center.
Things can get a bit more exciting in galaxies with three supermassive black holes, explains Avi Loeb, an astrophysicist who heads Harvard University's astronomy department. Three objects orbiting each other represents a far less stable system than two.
"You can get a slingshot ejection" of one of the three, which leaves the galaxy at speeds of several million miles an hour, says Dr. Loeb, who was not a member of the research team reporting the discovery. This mechanism could lead to a bevy of black holes in intergalactic space.
A formal report on the observations is set for publication in Thursday's issue of the journal Nature.
Black holes are objects with gravity so strong that not even light travels fast enough to escape their tug. Individual, massive stars can form small black holes at the end of their lives when they collapse and explode as supernovae. These can sport masses between three and several tens of times the mass of the sun. Supermassive black holes are roughly a million to billions of times the mass of the sun..
The discovery team, led by Roger Deane, an astronomer at the University of Cape Town in South Africa, notes that the trio of supermassive black holes appeared in the sixth system they observed.
If that's any indication of the abundance of such systems – and calculations Loeb and colleagues published two years ago suggest that it could be – "it's nice to see that nature follows that, Loeb says.
It's unclear how long-lived such close supermassive black hole binaries are, Dr. Taylor acknowledges. If they are long-lived, there should be a lot of them to find.
But the team's claim for the ubiquity of these pairs based on one find in six tries probably should be taken with a grain of stardust, he suggests.
Looking back on his own studies of binary supermassive black holes, "we found this one very interesting object about 10 years ago. That was one in 250 objects, so we though we'd open it up to 1,200 sources and we'll find six more. We didn't find any."
The new objects found by Deane and colleagues initially appeared as a quasar with some evidence that it might host two active supermassive black holes at its center.
Quasars are galaxies in which the region around their central black holes is emitting enormous amounts of radiation, indicating that the black holes are gorging on infalling dust and gas. These active galactic centers can be up to 100 times more luminous than the entire Milky Way.
Previous radiotelescope observations suggested a dual-core quasar. Deane and colleagues took the observations another step by using a widely dispersed set of interlinked radio telescopes in Europe and Puerto Rico, which allowed them to separate as distinct objects features that are only several tens of light-years apart – even from billions of light-years away.
This allowed them to separate one of the two active galactic centers into the close pair.
In doing so, the team noticed that the close pair produced outflows of gas whose radio signatures formed a S, the result of their mutual orbits and subtle changes to the orientation of their poles with time. Deane and colleagues suggest that as other astronomers hunt for close pairs, the S-shaped emissions should be a strong giveaway.
Many candidate systems meet the S requirement, adds Taylor of the University of New Mexico.