When galaxies collide, they tend to intermingle, ultimately forming a new, merged galaxy. And the supermassive black holes from the original galaxies’ cores should generally end up at the core of the new galaxy, according to current models. Some models predict that the two supermassive black holes could orbit each other, forming a black hole binary system. However, until recently, this has proved difficult to actually observe. Current instruments can’t resolve the difference between two supermassive black holes like these, which could be significantly less than a parsec apart.

But by using alternative methods, recent searches have found some promising candidates that could be supermassive black hole binary systems. In a new study, a team of researchers has reported a strong, clear signal from an extremely bright quasar that appears to be an example of a black hole binary. While this identification is still uncertain, the researchers conclude it’s the most plausible explanation of the behavior of that quasar.

Quasars

Quasars are simply extremely bright supermassive black holes, with the intense light originating from their jets and accretion disks. The jets, which emerge at each pole, are probably caused by their magnetic fields interacting with their spin and mass. The black hole also often has a disk of material falling in, called an accretion disk, that can produce a lot of light, since the infalling material is hot from friction.

If a quasar contained two black holes rather than one, detecting it would be tricky, since all these associated structures would interact. One way would be to look for quasars that are especially variable, meaning their intensity changes through a regular cycle. If there are indeed two black holes, their orbits could be causing the regular variations. The black holes’ jets would precess, meaning their orientations change over time in their orbits, explaining the variation. Perturbations in the holes’ accretion disks could also cause regular changes.

Light curves

To make their discovery, the research team examined data on quasars from the Catalina Real-time Transient Survey, which has been collecting data since 2009; they also looked at archived data going back to 1993. The data was used to track the quasars’ light curves, which show the object’s brightness over time. Light curves are useful for objects that have variable brightness, because they make it easy to pick out regular patterns in the variation.

Of all the quasars examined, 20 were good candidates for having regular variations. Of those, the strongest candidate is called PG 1302-102, which the researchers examined in detail. Its light curve resembled a sine wave, its light’s intensity gradually increasing and decreasing with a period of about 60 months.

While indications seemed strong that the variations in PG’s light curve were caused by the presence of two supermassive black holes, other mechanisms could still possibly create this pattern with a single supermassive black hole. The first of these is that there’s a single black hole in PG, and the light from PG is made up of contributions from both the black hole’s accretion disk and its jet—and the jet’s angle, due to precession, creates the variable pattern we observe.

While this could fit the data, it does have one major hurdle: if it was a single, precessing black hole with a jet, it should take from about a thousand to over a million years for one cycle. Remember that the observed period was about 60 months; the researchers concluded that this option is pretty unlikely.

A second possibility is that there’s a single black hole with an accretion disk that has a temporary hot spot. But getting a hotspot into a stable orbit with this period would require that the black hole has an unrealistically large mass.

It’s also possible that the single black hole’s accretion disk is warped, and that it’s partially eclipsed at regular intervals. If the disk is uneven, then it would be producing light unevenly, and so as the brighter section moves behind the black hole from Earth's perspective, it would get eclipsed and we’d see a variation in the quasar’s light. However, warped disks are thought to be the result of the interactions between two bodies in a binary. So if there’s only one supermassive black hole in PG, it raises the question of how the disk became warped in the first place.

To eliminate these and other alternative explanations, the researchers created computer simulations of different kinds of quasars—247,000 of them. Some of these had variable brightness due to various situations involving a single supermassive black hole, while others had two.

Out of all 247,000, only one simulated quasar was a good match for PG, and it was one with a supermassive black hole binary. In other words, out of all those possible systems, the one that looks the most like PG, at least in simulation, has two supermassive black holes. This provides a strong argument that the team’s conclusion is correct.

Supermassive binary

While the researchers conclude that a supermassive black hole binary system is the best candidate right now, the other explanations can't be ruled out definitively. Future observations should be able to distinguish among them, however.

Studying the relationship between PG’s host galaxy and its two neighboring galaxies, three and six kiloparsecs away from PG, respectively, could provide clues as to how the binary system formed in the first place. PG and these other galaxies are ellipticals and probably have supermassive black holes of similar mass.

While a binary would presumably form through the merger of two galaxies, piecing together the exact history of how that happened is important to our understanding of this system and to our understanding of the formation and evolution of galaxies in general.

If PG does turn out to be a supermassive black hole binary, the interactions of two bodies with such incredible masses and intense gravitational fields should be producing gravitational waves that might eventually be detectable. As gravity waves have yet to be directly observed, this would be a significant step forward.

Nature, 2014. DOI: 10.1038/nature14143 (About DOIs)