A galaxy has been discovered about 750 million light-years away, containing the largest pair of supermassive black holes we’ve ever seen. The total mass of this pair is 28 billion times greater than the mass of the Sun. While individual black holes exceeding this mass certainly exist, the pair hiding in the galaxy called B2 0402+379 is the most complex binary black hole we have found.

And they reveal some surprising features that help astronomers understand what happens to these giants when they come together. The evolution of black holes to supermassive sizes is a mysterious process whose variability is not known with certainty. Small black holes form from the collapsed cores of massive stars that have exhausted their nuclear fuel and can no longer shine. These stellar-mass black holes can collide with each other and grow larger, forming objects too massive to be formed by the collapse of the core.

There must be a method to enable black holes to grow to supermassive sizes (millions or billions of times the mass of the Sun). If small black holes can collide and merge, it seems logical to assume that large black holes must also be capable of a series of hierarchical mergers that will eventually create the giant black holes at the center of every galaxy.

But there is a potential problem, according to the theory. Black holes in binary systems merge and lose their orbital momentum, which they transfer to nearby gas stars flying to unknown destinations and lose in the form of gravitational waves.

As the orbital distance decreases, the region of space where they can transfer their energy also decreases. At a distance of about one parsec, or 3.2 light years, there is no room for further loss of momentum, so the orbital decay stops and stabilizes. This is known as the finite parsec problem.

B2 0402+379 may be a great example of the last parsec problem in action, according to a team led by Stanford University astrophysicist Tirth Surti. Researchers carefully examined archival data collected by the Gemini Multiple Object Spectrograph (GMOS) on the Gemini North telescope and conducted a new analysis that calculated the properties and behavior of two black holes embedded in the center of B2 0402+. 379.

Their results give us the mass of the binary system (28 billion solar masses) and reveal that the galaxy itself is a “fossil” galaxy cluster. B2 0402+379 was once a group of hanging galaxies; they eventually merged to form B2 0402+379.

A binary supermassive black hole is what is left of a cluster of black holes that collapsed into the galactic center and remained there. These two black holes are 7.3 parsecs, or 24 light years, away from each other. That’s not exactly the last parsec, and it’s not exactly the densest binary supermassive black hole we’ve ever seen.

But what makes this interesting is that the team’s analysis shows that orbital decay has stopped. These black holes have been separated from each other by this distance in a stable orbit for approximately 3 million years.

The discovery suggests that high mass may play a role in the ultimate parsec problem. The team believes that the binary system’s earlier orbital disruption had ejected so many stars from their surroundings that there were no longer any stars left to transfer their orbital momentum to. They’re doing pretty well right now.

“Galaxies with lighter black hole pairs generally appear to have enough stars and mass to merge quickly,” says astrophysicist Roger Romani of Stanford University.

“Because this pair was so heavy, it took a lot of stars and gas to do the job. “But the binary system cleared the central galaxy of such material, making it a dead end and available for our study.”

Alright what now? We know that black holes can somehow exceed the total mass of the binary system, but these supermassive giants seem to be quite rare. The B2 0402+379 nuclear binary appears to be very stable and there is no sudden loss of orbital momentum.

It could get a kick in the right direction from an injection of material that would follow another galactic merger by sending a third supermassive black hole into the group; but all the galaxies that made up the original cluster have already merged to form B2 0402+379, so this seems unlikely.

But there is another possibility. There may be some materials in the galaxy that could help the stuck union.

“We look forward to further investigation into the core of B2 0402+379 to see how much gas is there,” says Tirth. “This should give us more insight into whether supermassive black holes might eventually merge or whether they become stranded as a binary system.”