Comment by antognini

This system, OJ287, is perhaps the most important system we have for understanding what happens to supermassive black holes after a galaxy merger. This is the so-called "Last Parsec Problem."

When two galaxies merge, their supermassive black holes fairly rapidly sink to the center of mass of the newly combined galaxy via dynamical friction and enter into a slow orbit around each other. Over time, the SMBHs kick out interloping stars, which removes energy from the orbit and causes the two SMBHs to come closer together. If the SMBHs were able to get within ~0.1 parsecs of each other, gravitational wave radiation could take over and cause the orbit to shrink fairly rapidly and lead to the merger of the two SMBHs.

However, the theoretical models we have generally predict that at about 1 parsec, the SMBHs have kicked out all the stars in their neighborhood, so the process stalls out. In practice we don't observe many SMBH binary systems (OJ287 being the main exception), so there must be some mechanism that causes these systems to shrink from 1 pc to 0.1 pc. But we don't know what it is. The hope is that detailed studies of the orbit of OJ287 can provide some clues as to what that missing mechanism is.