One of the first questions you should always ask about science studies is “what is your sample size?” When talking about new drugs, expect numbers initially in the tens and then the hundreds. When dealing with computer models you may get thousands of simulations. With space science: we get super excited about a pair of pairs of supermassive black holes.

The sky is vast, the universe is deep, and finding examples of rare and interesting objects can be super hard. In discussing the evolution of galaxies, we tell the story of smaller galaxies and their central supermassive black holes coming together and merging, over and over, to build the most massive galaxies and supermassive black holes in the universe. 

The only problem is that while we see lots of galaxies merging, finding merging supermassive black holes has been harder, and until now, we only knew of one example – OJ 287 – of two tightly orbiting supermassive black holes. In that system, a 150 million solar mass black hole orbits an 18 billion solar mass black hole every twelve years. While one observation is enough for an astronomer to prove we’re probably right, it is better to have more than one example.

And now, a new system with two supermassive black holes with sizes around 100 million solar masses has been found. Cataloged as PKS 2131-021, the smaller of the two black holes appears to orbit the larger every 760.6 days or just over two years. This much smaller orbit puts these two systems closer together and closer to ultimately merging into a single system. In fact, these two supermassive black holes could merge in just 10,000 years.

Each of these systems gave itself away in different ways. OJ 287 has flares when its smaller supermassive black hole goes through the disk of material around the larger supermassive black hole. In PKS 2131-021, researchers don’t see anything as glaring as flares, but when researchers look at the systems jets, they see a sinusoidal pattern in the galaxy cores brightness in radio light that appears to correspond to a radio get moving back and forth. According to project mentor Tony Readhead: We knew this beautiful sine wave had to be telling us something important about the system,” Readhead says. “Roger’s model shows us that it is simply the orbital motion that does this. Before Roger worked it out, nobody had figured out that a binary with a relativistic jet would have a light curve that looked like this.

The Roger in question is co-author Roger Blandford who worked with lead author Sandra O’Neil on this project. O’Neil started this project as an undergraduate summer intern, pouring through data to find evidence of this object’s consistent variability in data as far back as 1976. With a long period, well-defined oscillation, Roger was able to piece together how the two supermassive black holes are orbiting. This work appears in The Astrophysical Journal Letters. As O’Neil puts it: Our study provides a blueprint for how to search for such blazar binaries in the future.

If we get lucky, maybe we can double our sample and find a whole four binary supermassive black holes out there, merging among the galaxies.

More Information

Caltech press release

NASA JPL press release

“The Unanticipated Phenomenology of the Blazar PKS 2131–021: A Unique Supermassive Black Hole Binary Candidate,” S. O’Neill et al., 2022 February 23, The Astrophysical Journal Letters