Free Astronomy Magazine July-August 2021

21 JULY-AUGUST 2021 ASTRO PUBLISHING A n artist’s im- pression cre- ated to visualize the concentration of black holes at the center of NGC 6397. In reality, the small black holes here are far too small for the direct observing ca- pacities of any ex- isting or planned future telescope, in- cluding Hubble. It is predicted that this core-collapsed glob- ular cluster could be host to more than 20 black holes. [ESA/Hubble, N. Bartmann] 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’Astro- physique de Paris set out to study the core of NGC 6397, they expected to find evidence for an “intermedi- ate-mass” black hole (IMBH). These are smaller than the super- massive 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 de- bated, although a few candidates have been found. To look for the IMBH, Vitral and Mamon analysed the positions and velocities of the cluster’s stars. They did this using previous estimates of the stars’ proper motions from Hubble images of the cluster spanning several years, in addition to proper motions pro- vided by ESA’s Gaia space observa- tory, which precisely measures the positions, distances and motions of stars. Knowing the distance to the cluster allowed the astronomers to translate the proper motions of these stars into velocities. “Our analysis indicated that the or- bits of the stars are close to random throughout the globular cluster, rather than systematically circular or very elongated,” explained Mamon. “We found 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 clus- ter,” added Vitral. 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 con- cluded 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 pro- posed that stellar remnants and in particular, stellar-mass black holes, could populate the inner regions of globular clusters. “Our study is the first finding to pro- vide both the mass and the extent of what appears to be a collection of mostly black holes in a core-col- lapsed globular cluster,” said Vitral. “Our analysis would not have been possible without having both the Hubble data to constrain the inner regions of the cluster and the Gaia data to constrain the orbital shapes of the outer stars, which in turn in- directly constrain the velocities of foreground and background stars in the inner regions,” added Mamon, attesting to an exemplary interna- tional collaboration. The astronomers also note that this discovery raises the question of whether mergers of these tightly packed black holes in core-collapsed globular clusters may be an impor- tant source of gravitational waves recently detected by the Laser Inter- ferometer Gravitational-Wave Ob- servatory (LIGO) experiment. !