Scientists have made an incredible discovery in a galaxy located 750 million light-years away. They have found the heaviest pair of supermassive black holes ever observed, with a combined mass of a staggering 28 billion times that of the Sun. These black holes, residing in a galaxy called B2 0402+379, are not only massive but also exhibit peculiar properties that are helping astronomers unravel the mysteries surrounding their formation and behavior.
The growth of black holes to such enormous sizes is still largely unknown. Small black holes are formed from the collapsed cores of massive stars that have exhausted their atomic fuel. These stellar mass black holes can grow by colliding with each other, resulting in objects too massive to form through the core collapse pathway. However, the process by which black holes grow to supermassive proportions, millions to billions of times the mass of the Sun, remains a mystery.
One theory suggests that if small black holes can collide and merge, then larger ones should be able to do the same. This would involve a series of hierarchical mergers that eventually produce the giant black holes found at the center of galaxies. However, there is a potential problem known as the final parsec problem. As black holes in binaries grow closer together, they shed their orbital momentum, transferring it onto nearby gas stars and losing it as gravitational waves. But at a certain distance, around a parsec or 3.2 light-years, there is no longer enough space to shed further momentum, causing the orbital decay to stall and stabilize.
The galaxy B2 0402+379 may provide an excellent example of the final parsec problem in action. A team led by astrophysicist Tirth Surti from Stanford University conducted a careful study of archival data collected by the Gemini Multi-Object Spectrograph (GMOS) on the Gemini North telescope. Their analysis revealed that the binary supermassive black hole in B2 0402+379 has a mass of 28 billion solar masses. Furthermore, they discovered that the galaxy itself is the remnant of a cluster of galaxies that merged together to form B2 0402+379.
The two black holes in this binary are separated by a distance of 7.3 parsecs or 24 light-years. Although not the final parsec, their stable orbit for over 3 million years is intriguing. The team believes that the previous orbital decay of the binary ejected many stars from their vicinity, leaving none onto which they can transfer their orbital momentum. This lack of nearby stars and gas has caused the black holes to become stuck in their current state.
Astrophysicist Roger Romani, also from Stanford University, explains that galaxies with lighter black hole pairs typically have enough stars and mass to drive the two black holes together quickly. However, due to the immense mass of this binary, it required a significant amount of stars and gas to bring them together. The binary has effectively cleared the central galaxy of such matter, leaving it stalled and accessible for study.
The question now is what happens next? While we know that black holes can grow larger than the combined mass of this binary, ultramassive black holes like these are rare. The nuclear binary of B2 0402+379 appears highly stable, with no immediate means of shedding orbital momentum. One possibility for future growth is the injection of material following another galactic merger, which could send a third supermassive black hole into the mix. However, since all the galaxies that made up the initial cluster have already merged to form B2 0402+379, this scenario seems unlikely.
Another potential solution lies within the galaxy itself. There may be some material present that could aid in merging the stalled black hole binary. Further investigations of B2 0402+379’s core will provide insight into the amount of gas present and whether the supermassive black holes can eventually merge or remain stranded as a binary. The research conducted by the team has been published in The Astrophysical Journal, and scientists eagerly await future discoveries that will shed more light on the mysteries of supermassive black holes.