Concentration of smaller black holes in the cluster NGC 6397

Globular clusters are extremely dense stellar systems & host stars together

• The 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
• Small black holes were discovered instead of one massive black hole at the heart of the cluster

Summary of the discovery

•  Scientists were expecting to find an intermediate-mass black hole at the heart of the globular cluster NGC 6397, but instead they found evidence of a concentration of smaller black holes lurking there
• New data from the NASA/ESA Hubble Space Telescope have led to the first measurement of the extent of a collection of black holes in a core-collapsed globular cluster
• Globular clusters are extremely dense stellar systems, in which stars are packed closely together
• They are also typically very old — the globular cluster that is the focus of this study, NGC 6397, is almost as old as the Universe itself
• It resides 7800 light-years away, making it one of the closest globular clusters to Earth
• Because of its very dense nucleus, it is known as a core-collapsed cluster
• When Eduardo Vitral and Gary A. Mamon of the Institut d’Astrophysique de Paris set out to study the core of NGC 6397, they expected to find evidence for an “intermediate-mass” black hole (IMBH)
• These are smaller than the supermassive black holes that lie at the cores of large galaxies, but larger than stellar-mass black holes formed by the collapse of massive stars
• IMBH are the long-sought “missing link” in black hole evolution and their mere existence is hotly debated, although a few candidates have been found

Key findings of the Discovery

•  The orbits of the stars are close to random throughout the globular cluster, rather than systematically circular or very elongated
• Very strong evidence for invisible mass in the dense central regions of the cluster, but we were surprised to find that this extra mass is not point-like but extended to a few percent of the size of the cluster
• This invisible component could only be made up of the remnants (white dwarfs, neutron stars, and black holes) of massive stars whose inner regions collapsed under their own gravity once their nuclear fuel was exhausted
• The stars progressively sank to the cluster’s centre after gravitational interactions with nearby less massive stars, leading to the small extent of the invisible mass concentration
• Using the theory of stellar evolution, the scientists concluded that the bulk of the unseen concentration is made of stellar-mass black holes, rather than white dwarfs or neutron stars that are too faint to observe
• Two recent studies had also proposed that stellar remnants and in particular, stellar-mass black holes, could populate the inner regions of globular clusters
• First finding to provide both the mass and the extent of what appears to be a collection of mostly black holes in a core-collapsed globular cluster

What are Black Holes?

•  A black hole is a region of space time where gravity is so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from it
• The theory of general relativity predicts that a sufficiently compact mass can deform space time to form a black hole
• The boundary of the region from which no escape is possible is called the event horizon
• Although the event horizon has an enormous effect on the fate and circumstances of an object crossing it, according to general relativity it has no locally detectable features
• In many ways, a black hole acts like an ideal black body, as it reflects no light
• Moreover, quantum field theory in curved space time predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass
• This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it essentially impossible to observe directly