They used the method to measure the gigantic black hole in the galaxy with the catalog number NGC 4526: it has the impressive mass of around half a billion suns. The astronomers used the motion of molecular gas, in this case carbon monoxide (CO), swirling around the black hole as a "scale". From the motion of the gas clouds the gravity of the black hole can be determined and from that again its mass.
Astronomers have previously weighed black holes in a similar way, by measuring the motion of stars or ionized gas in the vicinity of the black hole. However, this only works for certain types of galaxies. By observing molecular gas, on the other hand, many more types of galaxies should be accessible.
With the increasing detail of observational instruments, researchers expect that their method will be able to determine the masses of rough black holes in hundreds of galaxies in our extended cosmic neighborhood. Instruments such as the international atacama large millimeter array ALMA, which is currently under construction, could measure the mass of a black hole in only five hours. This information, obtained in a unified way, will allow scientists to make more precise predictions about the co-evolution of galaxies and their central black holes.