Star-killer. Retrograde black holes may produce the gigantic jets that suppress star making in galaxies.


Are Backward Black Holes the Key to Galaxies' Biggest Blasts?

If you're planning to be a star, steer clear of supermassive black holes that spin in the opposite direction from their surrounding galaxies. Astrophysicists now think that, for reasons predicted by Einstein's general theory of relativity, such black holes tend to create unusually energetic jets of particles—powerful enough to interfere with the star-making process not only within the galaxy but also across intergalactic space. The findings, if confirmed, would solve a longstanding mystery about why some galaxies produce much bigger—and therefore more energetic—particle jets than others do.

Every massive galaxy contains at its center at least one supermassive black hole. These monsters can pack the mass of up to a billion suns or more into a space about the size of our solar system. All of that matter creates an immense amount of gravity, enough to suck in any gas, dust, planet, star—even light—unfortunate enough to get caught within its sphere of influence. The black hole's gravity helps to compress and flatten the surrounding matter into a spinning pancake called an accretion disk.

Along with that disk, a supermassive black hole also spins. Except that its spinning severely warps space and time, a process that liberates enormous amounts of energy. Much of that energy can flow in the form of gigantic, magnetically driven, high-speed particle jets that spew from the north and south poles of the black hole's spin axis. Like cosmic tsunamis, the jets sweep away everything in their path for hundreds of thousands of light-years—including the dust and gas that normally would have congealed into new stars and planets. If a neighboring galaxy happens to be in the way, a jet can disrupt its star-making activity as well.

Astronomers have observed such jets in many galaxies, nearly all of them in the very distant universe, seen as it existed billions of years ago. But some galaxies display jets of much less energy, and others show no jets at all. Why the discrepancy?

The answer has stumped astrophysicists for more than a decade. Some theorists proposed that the power of galactic jets depended on the speed of a black hole's spin: The faster the spin, the stronger the jet. But then they found galaxies with fast-spinning supermassive black holes that produced no jets at all. Obviously, some other factor was at work.

So theorists proposed that the big jets could be created by black holes spinning "backward"—in the opposite direction from their accretion disks. The physics of these so-called retrograde black holes, they calculated, would create more powerful magnetic fields—and magnetic fields are what drive the jets.

Eventually, a few galaxies were found whose radio emissions suggested that they could be harboring retrograde supermassives, and those galaxies indeed displayed powerful jets, but until now no one had figured out a reason for the connection. In an upcoming issue of the Monthly Notices of the Royal Astronomy Society, a team of astrophysicists claims to have found the answer—and it's relatively simple. Studying the galaxies suspected of hosting retrograde supermassive black holes, they found that the galaxies with the biggest jets were all located at great distances, meaning that they existed when the universe was much younger. As the distances to the galaxies diminish, the researchers argue, so do the power of their jets—and the likelihood that their central black holes are retrograde.

The team inferred that, over time, the inertia of the surrounding galaxy and accretion disk wears down the spin of the warped space created by a retrograde supermassive. Eventually, they concluded, retrograde black holes actually reverse their spin and begin rotating in the direction of their galaxy, thereby losing the energy to create particle jets. But for a while, the relativistic dynamo of spinning warped space, meeting incoming matter trapped by the black hole's gravity, blasts out so much energy that even an exploding star would be minuscule by comparison. "Our findings tell us that general relativity is a fundamental driver of galaxy evolution via black hole spin," says astrophysicist and co-author David Garofalo of NASA's Jet Propulsion Laboratory in Pasadena, California.

As to why black holes become retrograde in the first place, Garofalo says that he suspects mergers of supermassive black holes in the early universe are responsible. Confirming the idea will require further research, but in the meantime, he says, the findings imply "that general relativity is important in the evolution of galaxies and thus to the universe as we know it—a rather surprising idea."

The researchers explain "a lot of observational facts with a single, simple hypothesis—and that's a good thing," says astrophysicist Steven Willner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. On the other hand, he says, supermassive black holes and their host galaxies are influenced by many complicated processes. So "no single idea is going to be the whole story," he says. "We'll need some more work to see how important this idea is."

Astrophysicist Christopher Reynolds of the University of Maryland, College Park, says it's a new idea that retrograde accretion is particularly effective at tapping into relativistic energy and forming powerful jets. If the researchers are correct, he says, "this maybe is the clue we've been looking for to understand why powerful [jets] are found in certain environments."