Supermassive black holes are often surrounded by disks of material that slowly but surely falls beyond the boundary, or event horizon, feeding the black hole within. Black holes spin and their disks usually spin with them, but astronomers have discovered a disk stubbornly going the other way around. The finding is reported in The Astrophysical Journal Letters.
Galaxy NGC 1068 is a spiral galaxy 47 million light-years from Earth. At its core, there is a supermassive black hole, almost 17 million times the mass of our Sun. Astronomers recently employed the Atacama Large Millimeter/submillimeter Array (ALMA) to better study its environment and discovered the presence of two accretion disks (rotating disks of matter).
“Thanks to the spectacular resolution of ALMA, we measured the movement of gas in the inner orbits around the black hole,” lead author Dr Violette Impellizzeri, from the National Radio Astronomy Observatory (NRAO), said in a statement. “Surprisingly, we found two disks of gas rotating in opposite directions.”
The counter-rotating disk stretches between 4 and 22 light-years from the black hole, while the donut of gas spinning the right way stretches between 2 and 4 light-years from it.
“We did not expect to see this, because gas falling into a black hole would normally spin around it in only one direction,” said Impellizzeri. “Something must have disturbed the flow because it is impossible for a part of the disk to start rotating backward all on its own.”
The counter-rotating disk could be gas clouds that fell from the host galaxy in a peculiar direction or it may be the result of a minor merger event that happened in the past. There is evidence to suggest that NGC 1068 interacted with a small galaxy several billion years ago.
The work can also tell us about supermassive black hole evolution. The team believes that the presence of counter-rotating disks affects how quickly a black hole gains mass, and maybe this is how the earliest supermassive black hole became so big in such a short time.
“Counter-rotating gas streams are unstable, which means that clouds fall into the black hole faster than they do in a disk with a single rotation direction,” said Impellizzeri. “This could be a way in which a black hole can grow rapidly.”
The outer disk is currently in a stable orbit but once the two disks interact (which might happen in a few hundred thousand years), the system will become unstable and the material will fall into the black hole.