Almost all galaxies have supermassive black holes (SMBHs) in their centres. These cosmic giants are millions to billions of times more massive than the Sun, and play a key role in shaping the formation and evolution of galaxies. Galaxies grow through collisions and mergers of smaller galaxies, and when pairs of galaxies merge their SMBHs slowly sink towards the centre of the merged galaxy. As they get closer together the “friction” dragging them inwards (actually gravitational encounters with stars) becomes progressively less and less efficient, and this process ultimately stalls when the SMBHs reach separations of around a parsec. However, no pairs of binary SMBHs are seen at these separations (despite many years of searching), suggesting that the process(es) that drive SMBH mergers are much more efficient than theory suggests. This long-standing discrepancy between theory and observations is known as the “last parsec problem”.
The most popular solution to this problem is to appeal to a gaseous accretion disc around the SMBH binary. These discs are commonly observed around single SMBHs (where they appear as quasars), and they allow SMBHs to grow by funnelling material down towards the event horizon. However, it is well-known that if the binary and disc are aligned (i.e., they orbit in the same plane and rotate in the same direction), then the disc cannot shrink the binary’s orbit quickly enough. The Theoretical Astrophysics Group in Leicester has recently pioneered the idea that misaligned accretion discs may be the key mechanism driving SMBH mergers. The interaction between an accretion disc and a misaligned SMBH binary is very complicated, and can only be studied using large computer simulations. The power of DiRAC-2 allowed us to build detailed simulations of how accretion discs form and evolve around SMBH binaries (see Figure 1, adapted from Dunhill et al 2014. Animations from these simulations are available here). These are the first models to study the disc formation process self-consistently (previous studies have simply assumed that a disc is present), and to look in detail at how the gravity of the disc shapes the evolution of the binary. As with previous studies, we find that prograde discs (i.e., disc which rotate in the same direction as the binary) do not shrink the binary’s orbit efficiently. However, when discs form in retrograde configurations (i.e., with the gas orbiting in the opposite direction to the binary) their interaction with the binary is much more efficient. Our simulations demonstrated that repeated retrograde accretion events can drive mergers between SMBH binaries on short time-scales, and this process offers an elegant potential solution to the last parsec problem.