Astronomers discover concentration of small black holes

In an unexpected finding, astronomers have discovered a concentration of smaller black holes, instead of one massive black hole, lurking at the heart of the globular cluster NGC 6397.

Globular clusters are extremely dense stellar systems, which host stars that are closely packed together.

These systems are also typically very old – the globular cluster at the focus of this study, NGC 6397, is almost as old as the universe itself.

This cluster resides 7,800 light-years away, making it one of the closest globular clusters to Earth. Due to its very dense nucleus, it is known as a core-collapsed cluster. At first, astronomers thought the globular cluster hosted an intermediate-mass black hole. These are the longsought “missing link” between supermassive black holes – many millions of times our Sun's mass – that lie at the cores of galaxies, and stellar-mass black holes – a few times our Sun's mass – that form following the collapse of a single massive star. Their mere existence is hotly debated. Only a few candidates have been identified to date.

"We found very strong evidence for an invisible mass in the dense core of the globular cluster, but we were surprised to find that this extra mass is not 'point-like' (that would be expected for a solitary massive black hole) but extended to a few percent of the size of the cluster," said Eduardo Vitral of the Paris Institute of Astrophysics (IAP) in Paris, France. –IANS