Hubble exposes concentration of small black holes

Globular clusters are extremely dense galaxies in which stars are now packed together. They are usually very old – the spherical group that is the focus of this study, NGC 6397, is almost as old as the universe itself. It is 7800 light-years away, making it one of the closest globular clusters to Earth. Because of its very dense core, it is known as a group that collapsed.

When Eduardo Vitral and Gary A. Mamon of the Institut d’Astrophysique de Paris went to study the core of NGC 6397, they expected to find evidence for an ‘intermediate mass’ black hole (IMBH). It is smaller than the supermassive black holes that lie at the core of large galaxies, but larger than black mass stars formed by the collapse of massive stars. IMBH is the long-sought missing link in the evolution of black holes, and their mere existence is hotly debated, although a few candidates have been found.

To search for the IMBH, Vitral and Mamon analyzed the positions and velocities of the stars of the cluster. They did so using previous estimates of the stars’ true motions of Hubble images of the cluster that stretched over several years, in addition to the true motions offered by ESA’s Gaia space station, which accurately measure the positions, distances and motions of stars. With the knowledge of the distance to the group, the astronomers were able to translate the correct motions of these stars into velocities.

“Our analysis indicated that the orbits of the stars are close to random throughout the spherical group, rather than systematically circular or very elongated,” Mamon explained.

“We found very strong evidence for invisible mass in the dense central regions of the group, but we were surprised to see that this extra mass is not point-like, but extends to a few percent of the size of the group,” “Vitral added.

This invisible component could only consist 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 depleted. The stars gradually descended to the center of the cluster after gravitational interactions with nearby less massive stars, resulting in the small degree of invisible mass concentration. Using the theory of stellar evolution, scientists have come to the conclusion that most of the unseen concentration consists of black masses of stellar mass, rather than white dwarfs or neutron stars that are too faint to observe.

Two recent studies have also suggested that star remains, and especially black holes, may populate the inner regions of spherical clusters.

“Our study is the first finding that provides both the mass and extent of a collection of mostly black holes in a spherical spherical group,” Vitral said.

“Our analysis would not have been possible without both the Hubble data restricting the inner regions of the group and the Gaia data to limit the orbital shapes of the outer stars, which in turn indirectly affect the velocities of the front and back. background stars in the interior, “Mamon added, testifying to an exemplary international collaboration.

The astronomers also note that this discovery raises the question of whether mergers of these densely packed black holes in nucleated spherical clusters may be a major source of gravitational waves recently detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) experiment. .