Supermassive black holes at the centers of galaxies are one of the most active fields of research in astronomy. In order to accumulate their enormous masses, they must merge with each other. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn has found direct evidence of two supermassive black holes in the galaxy Markarian 501, which orbit each other very closely. This could be the first time that a pair has been detected that is about to merge. This provides a unique opportunity to better understand a central process in galaxy evolution.

The findings suggest that there is a supermassive black hole at the center of almost every large galaxy, with a mass millions or even billions of times greater than that of our sun. It is still unclear exactly how they can reach such enormous masses. Collecting (accreting) gas from the surrounding area alone would take too long, so it is likely that they have to merge with other massive black holes. Galaxy collisions have been observed throughout our universe. It is thus very likely that the supermassive black holes at the centers of these colliding galaxies also merge, first orbiting each other ever closer and ultimately coalescing into one.