The mass of a supermassive black hole correlates closely with the velocity dispersion of its host galaxy's bulge.

These giant black holes do not live in isolation. They grow in tandem with the galaxies that house them. By measuring how fast stars move in the central bulge of a galaxy, astronomers found a tight link to the mass of the central black hole. This correlation suggests that black holes and their host galaxies shape each other's growth over billions of years.