Hubble detects a cluster of small black holes

Hubble detects a cluster of small black holes. In the centre of the globular cluster NGC 6397, astronomers have discovered what they did not expect, instead of one massive black hole, several smaller black holes are hidden there, according to the press service of NASA.

Globular clusters are incredibly dense star systems. These systems are also usually ancient. Thus, the globular cluster NGC 6397 is almost as old as the Universe itself. It is also one of the closest groups to us: NGC 6397 lies 7,800 light-years from Earth.

At first, astronomers believed that this globular cluster contains a black hole of average mass. It would be the “missing link” between supermassive black holes (many millions of times the mass of our Sun) lying in the cores of galaxies, and black holes of stellar mass (several times the mass of our Sun), which form after the collapse of a single massive star.

The existence of such black holes is still under debate: to date, only a few candidates have been identified.

Scientists have received robust evidence that there is the invisible mass in the dense core of the globular cluster NGC 6397. It turned out that this additional mass is not concentrated pointwise: it is stretched in space, which occupies a few per cent of the cluster’s size.

The researchers used data from the Hubble Telescope and the European Space Agency’s Guy Space Observatory to determine stars’ speeds within the cluster. They concluded that the invisible component could only consist of the remnants of massive stars (white dwarfs, neutron stars, and black holes), given its mass, extent and location.

These stellar “corpses” gradually descended towards the cluster’s centre after gravitational interaction with nearby less massive stars. This “star pinball” game is called “dynamic friction,” where momentum exchange causes heavier stars to separate in the cluster’s core. In contrast, lower-mass stars migrate to the periphery of the collection.

“We used the theory of stellar evolution to conclude that most of the extra mass we found was in the form of black holes,” the authors of the work noted. And they added that the discovery raises the likelihood that mergers of these densely packed black holes into globular clusters could be an essential source of gravitational waves vibrating in spacetime.