Data from NASA’s Galaxy Evolution Explorer (GALEX) and the Hubble Space Telescope has confirmed the presence of a pair of supermassive black holes orbiting each other so closely that they're moving at relativistic speeds—a significant fraction of the speed of light.

Supermassive black holes are expected to come in pairs pretty often. That’s because every galaxy has its own supermassive black hole, and galaxies often merge, bringing the two together. These mergers are very slow processes that distort both galaxies until their stars settle into new orbits (a process known as "violent relaxation"). While this is happening, extremely heavy objects, such as supermassive black holes, will tend to move in toward the center of the new galaxy. The new galaxy would end up with two supermassive black holes, one from each original galaxy, orbiting each other at its core.

Objects have been observed which look a lot like supermassive black hole binaries, matching the prediction. These objects have a lot of mass—billions of times the mass of the Sun, as we’d expect from a pair of supermassives—and they’re periodic, meaning the amount of light the object produces rises and falls with a predictable time period.

(It may at first seem contradictory to think of light coming from a black hole, an object from which no light can escape. The light doesn’t actually come from the black hole itself but from matter falling in, which produces light from the incredible friction it experiences as it speeds up, spiraling into the black hole).

The candidate

One likely black hole binary, PG 1302-102, was first observed last year by ground-based telescopes. A new study confirms this tentative identification. The estimated masses of the two black holes, along with the orbital period, allow us to estimate how close they are. In this case, the answer is "really close."

The inferred distance between them is somewhere between .007 and .017 parsecs, which is not much bigger than the diameter of the Solar System. That’s astoundingly close for two objects of this size—so close that there’s been debate in the scientific community as to how that's possible (See sidebar). To maintain their orbit, the black holes have to be whipping around at relativistic speeds.

The Final Parsec Problem PG 1302-102 is a system that has found a way around the “Final Parsec Problem,” a major unsolved issue in astrophysics. As such, it could provide important clues in understanding how to resolve this issue. The problem is that while it’s easy to understand how two supermassive black holes could come into orbit with one another, it’s not as easy to model how they cross the final parsec of space between them to merge. “[Known mechanisms] can get the BHs [black holes] close but not close enough (past the final parsec of separation) for gravitational waves to finish the job of merging the BHs,” Daniel D’Orazio of Columbia University told Ars. Current models have them falling into relatively stable orbits more than one parsec away from each other. So there must be some other mechanism to help them fall in closer. Once they get in beyond the “finish line” of one parsec, another known mechanism takes over to complete their merging: gravitational waves. The black holes begin to emit large amounts of gravitational waves as they get closer to one another. These waves carry energy away from the system as they leave. This slows down the black holes, causing them to fall in toward each other bit by bit. But for this to occur, the black holes have to be within a critical distance. And we're not sure what gets them there. For now, the identity of this in-between mechanism remains a mystery. “There are definitely good ideas of how it can be solved (gas for example), but a definitive solution has not come about,” said D’Orazio. As to whether there are any indications of what that mechanism may be, “I don't have anything definitive to say at this point on [that], except that it will be interesting to continue modeling such systems and find out,” D’Orazio told Ars.

These incredible speeds allow for a process called relativistic beaming: as one of the black holes moves away from us, it’s moving so fast that its light actually dims. As it moves toward us, its light brightens. This effect is too small to be noticeable at normal speeds, but at significant fractions of the speed of light, it becomes observable.

“When any light emitting object moves toward you, the object appears brighter and bluer. When it moves away from you, it appears dimmer and redder,” Daniel D’Orazio of Columbia University and the lead author of the paper told Ars. “However, the speeds of objects we encounter on Earth everyday are not large enough for us to notice this effect called relativistic beaming (a prediction of special relativity). But when a light-emitting object moves close to the speed of light, the effect is very noticeable.”

Part of this is due to the Doppler effect, by which wavelengths get longer when their source is moving away from you and shorter when they’re moving toward you (think of a police siren speeding away down the road). This causes any ordinary star to undergo red- and blue-shifts in different parts of its orbit. The other part of the effect is due to the incredible speed, which causes the brightening and dimming.

Digging through data

To confirm it was a black hole pair exhibiting relativistic beaming, the researchers needed data on the patterns of light extending over many years. That’s because the pair take about five years to complete one orbit, making it tricky to get the full pattern. Luckily, GALEX and Hubble both had data going back 20 years in different wavelengths, providing a treasure trove.

"We were lucky to have GALEX data to look through," said co-author David Schiminovich of Columbia University in New York. "We went back into the GALEX archives and found that the object just happened to have been observed six times."

Both telescopes captured data in the ultraviolet portion of the spectrum, which showed the sudden brightening and darkening due to relativistic beaming. "It's as if a 60-Watt light bulb suddenly appears to be 100 Watts," said D'Orazio. "As the black hole light speeds away from us, it appears as a dimmer 20-Watt bulb."

The use of ultraviolet in particular was important because it helped confirm the researchers’ prediction. “We have predicted that UV light should have a larger amplitude of variability than the light in the optical part of the electromagnetic spectrum, and this is what we find in the data,” D’Orazio told Ars. In other words, the brightness varies more in UV than in visible light, confirming predictions for a binary black hole system with relativistic beaming. “The point being that relativistic beaming gives a good case for what we are seeing in PG 1302, and it looks like what we see is caused by BHs orbiting very close to each other very fast.”

Spiral into the future

The confirmation is an important one for researchers trying to piece together the results of galaxy mergers. Many known galaxies are the result of these mergers, making this significant to understanding structure in the Universe.

The two supermassive black holes of PG 1302-102 will not remain in their absurdly fast orbit forever. Rather, they’re expected to spiral into one another and merge in about a million years. The system could therefore provide important clues about how such mergers work.

As the black holes spiral into one another, their violent motions create waves in spacetime that propagate outward, known as gravitational waves. These waves could in principle be detected, though none have been as of yet. (Researchers hope that some will be soon, as the newly upgraded LIGO came back online this month, twice as sensitive as before). Sadly, PG-1302-102 is not yet far enough along in its merging to produce detectable gravitational waves. We’ll have to wait close to a million years for that.

The researchers’ technique could aid in the discovery of more supermassive black holes with relativistic beaming effects—which could in turn lead to a better understanding of the formation of such systems.

“The fact that black holes can make it to this regime is interesting because of something called the final parsec problem,” D’Orazio told Ars. “Which says that based on our understanding of black hole binary formation, it is very hard to get the BHs this close. If we find evidence for BHs this close, then we learn something about how BHs and galaxies evolve together.”

In the meantime, researchers now have a "laboratory" of sorts in PG 1302-102: a system they can study to learn about the complexities of galaxy and black hole formation.

Nature, 2015. DOI: doi:10.1038/nature15262 (About DOIs)