Moons orbit planets, planets go around stars, and stars in galaxies orbit supermassive black holes. And the chain usually ends here because supermassive black holes—supermassive as they are—sit tight in their place, sucking in everything around them. At least that’s what astronomers thought until now.
But recently, scientists from the Centre of Astrophysics, Harvard and Smithsonian have found a peculiar case of a “restless” one wandering through space. Maybe everyone’s just tired of sitting in one place through 2020 and is revenge-travelling?
“We don't expect the majority of supermassive black holes to be moving; they're usually content to just sit around,” said Dominic Pesce, an astronomer at the Center for Astrophysics who led the study, that was published in The Astrophysics Journal on March 12. “They're just so heavy that it's tough to get them going. Consider how much more difficult it is to kick a bowling ball into motion than it is to kick a soccer ball — realizing that in this case, the 'bowling ball' is several million times the mass of our Sun. That’s going to require a pretty mighty kick.”
Pesce and his fellow researchers had been comparing the velocities of supermassive black holes for the last five years. “We asked: Are the velocities of the black holes the same as the velocities of the galaxies they reside in?” he said. “We expect them to have the same velocity. If they don't, that implies the black hole has been disturbed.”
The team surveyed ten distant galaxies and the supermassive black holes at their cores. They studied black holes that contained water within their accretion disks—the spiral structures that spin inward towards the black hole.
Masers are produced when the water orbits around a black hole and produces a laser-like beam of radio light. Pesce believes that masers can help measure a black hole’s velocity precisely when studied with a combined network of radio antennas using a technique called very long baseline interferometry (VLBI).
This technique showed that nine of the ten supermassive black holes were at rest—but one, whose mass is about three million times that of our Sun, stood out and seemed to be in motion.
With follow-up observations from Arcebio and Gemini observatories, scientists have confirmed that this on-the-go black hole is moving at the speed of about 177,027.84 kilometres per hour inside the galaxy J0437+2456 but they don’t know what’s causing it to move in an unusual way. The team suspects there are two possibilities.
One could be that this a result of two supermassive black holes merging. “The result of such a merger can cause the newborn black hole to recoil, and we may be watching it in the act of recoiling or as it settles down again,” says Jim Condon, a radio astronomer at the National Radio Astronomy Observatory, who was involved in the study.
But there's another unexplored possibility: The black hole may be part of a binary system, and orbiting a shared centre of gravity along with a fellow supermassive black hole “Despite every expectation that they really ought to be out there in some abundance, scientists have had a hard time identifying clear examples of binary supermassive black holes,” Pesce said. “What we could be seeing in the galaxy J0437+2456 is one of the black holes in such a pair, with the other remaining hidden to our radio observations because of its lack of maser emission.”