The galaxies we see in the present-day Universe were built through the merger of smaller ones. Almost all of the galaxies contain supermassive black holes at their cores. Basic logic would suggest that the mergers would also have placed supermassive black holes in close proximity at the cores of galaxies. What's less certain is what happened to them once they were brought together.

Ultimately, the fate of these black holes will be to merge. But if the process is slow enough, we should see a large number of binary supermassive black holes lingering in the cores of galaxies, producing gravity waves as they interact. We'd love to detect those gravity waves, but it's hard to justify building the appropriate detector until we know they're out there, which means we need to determine whether supermassive black hole binaries are common.

Today, an international team of researchers is announcing that they've found a triple black hole system, with two of the objects forming a tight binary system. The good news for gravity waves is that they found this system in one of the first handful of systems they checked, and they suggest that the signs of these systems might be relatively easy to spot.

The new system, which has the catchy name SDSS J150243.09+111557.3, was initially identified as a quasar, a bright object powered by the violent jets of matter that are driven by the black hole. Quasars are common, but this one had an unusual feature: energetic emissions from oxygen displayed two distinct peaks, slightly shifted relative to each other. One possible explanation for that feature is that there are actually two sources of these oxygen emissions moving at more than 650 kilometers a second relative to each other. (One source would be blue shifted, one red shifted, resulting in two distinct peaks.) Imaging in the near infrared confirmed that there are two distinct sources separated by 7,400 parsecs (each parsec is a bit over three light years).

To get a closer look, the researchers imaged the system with the European VLBI Network (VLBI stands for Very Long Baseline Interferometry), a collection of radio telescopes that can be operated in a way that allows them to act as a single, enormous imaging device. That imaging revealed that one of the two sources they knew about was actually composed of two distinct sources of radio emissions, both of them supermassive black holes.

The two are close enough together that orbital interactions have twisted the jets of one black hole into an S-shaped curve. The authors estimate that the separation between the two is only 140 parsecs, making them the second-closest pair of supermassive black holes we've ever spotted. This is also only the fourth triple black hole system that's been detected.

The authors think that the S-shaped jets might be a diagnostic indication of these closely spaced black hole systems. They looked at a total of only six galaxies before spotting this pair, which they take as an indication that similar systems may be common—a contrast to previous searches, which had found very few.

The researchers conclude by arguing that it's worth continuing the search for more systems with multiple black holes. Identifying them will help us determine how common gravity waves of different frequencies are, which will help us design the next generation of detectors.

Nature, 2014. DOI: 10.1038/nature13454 (About DOIs).