The cluster NGC 6397 is one of 150 spherical clouds of ancient stars that orbit the Milky Way and may predate its formation. It is 13 billion years old and contains about 250,000 stars, all old, small and faint. The brightest and heaviest stars in the cluster long ago burned up their lives and found their fate as black holes or other products of stellar disintegration.
So the cluster is a likely candidate to host an intermediate mass black hole. In fact, previous studies had suggested that a black hole of about 600 solar masses anchored the center of NGC 6397.
To investigate that idea, Dr. Mamon and his student turned to high-resolution observations of the motions of individual stars in the cluster, obtained by the Hubble Space Telescope and Gaia. The faster the stars are moving, the greater the gravitational force, and therefore there must be more mass to keep them in the cluster.
In all, 1,905 stars from the Gaia catalog and 7,209 stars from Hubble were evaluated. As it turned out, they were in fact under the gravitational influence of an invisible mass. But instead of tightly circling a single dark point, the stars were moving in all directions, suggesting that any influencing dark masses were not concentrated, but spread out. There was no sign of a gigantic black hole.
“Our analysis indicated that the orbits of the stars are almost random throughout the globular cluster, rather than being systematically circular or very elongated,” Dr. Mamon said in an email.
The motions of those stars offered evidence of a dark mass equal to 1,800 suns spread across a region, a cloud within the cloud, about a third of a light-year across. Sharing that space are about 40,000 ordinary stars, luminous but very light. Based on models of stellar evolution, Mamon said, about two-thirds of this dark matter would be black holes, with an average mass of 20 suns. The rest of the dark material would be the remains of dead stars, such as white dwarfs or neutron stars.
What will happen next is unclear, the astronomers said. Random mergers between black holes could cause them to lose mass in the form of gravitational waves. Such mergers could also drive some black holes out of the cluster. And gravitational interactions with less massive stars in the cluster could cause them to slow down and sink into the center of the cluster, in a process called “dynamic friction.”