The second closest globular cluster to Earth, NGC 6397, may host a concentration of unseen mass mostly composed of black holes. These results were obtained by application of sophisticated statistical tools to the positions and velocities of the visible stars in the cluster, observed with different telescopes in space and on the ground.

High resolution image of the globular cluster NGC 6397 Figure 1: Image of globular cluster NGC6397, obtained by combining images in two optical filters taken by the Hubble Space Telescope. Credits: NASA, ESA, T. Brown, S. Casertano et J. Anderson (STScI)

When a star exhausts its sources of nuclear fusion energy, it leaves behind a “dead star”, often a white dwarf star (that is prevented from collapsing under its own weight by the pressure of electrons); or, if it starts more massive than roughly 8 times the Sun's mass, a neutron star (that is prevented from collapsing under its own weight by nuclear forces); both types of objects emit very little light. In contrast, the gravity of a star above 20 times the mass of the Sun is so intense that nothing can stop its collapse, and it forms a black hole: a singularity where all the mass is concentrated, surrounded by a small virtual sphere called the “event horizon” from which not even light can escape. These objects are called stellar-mass black holes.

Black holes have been in the imagination of scientists since the beginning of last century. Initially, they were only a theoretical solution to Einstein’s new equations of relativity. Significant efforts have been made to detect these objects. This is challenging because no light can escape from the inside of a black hole. Therefore, it is impossible to directly detect such an object, and one must instead infer its presence from the matter surrounding it, which would have different properties and emissions if no black hole was present.

Recently, two scientific collaborations have made breakthroughs in black hole physics. First, since 2015, unequivocal evidence of black hole mergers has come from the gravitational waves detected by the LIGO (Laser Interferometer Gravitational-Wave Observatory) collaboration, as mergers of very massive and close objects produce disturbances in space-time. Then, in 2019, the Event Horizon Telescope, a collaboration of several radio telescopes around Earth, imaged a black hole at the center of the giant Messier 87 galaxy, outlined by its hot surrounding gas[1]. In the centers of galaxies, black holes are expected to be supermassive, with masses several millions to billions times that of the Sun.

Between the stellar-mass black holes and the supermassive ones, astronomers expect the existence of intermediate-mass black holes, with masses between 100 and 100,000 times that of the Sun. But very few members of this class of black holes have been robustly confirmed[2],[3]. Intermediate-mass black holes are expected to be found inside globular star clusters, which are tight near-spherical collections of up to a million stars. Some of these systems are almost as old as our Universe itself. The central part (or “core”) of these star clusters is so dense that stars lie 100 to 1,000 times closer to one another than the Sun and its neighboring stars. Given these high densities, stars are expected to interact and collide in the cluster core, which may in turn lead to the formation of intermediate-mass black holes.

Doctoral student Eduardo Vitral and his advisor Gary Mamon, both at the Institut d’astrophysique de Paris (IAP), investigated one of the closest globular clusters in our galaxy, NGC 6397 (see Figure 1). This cluster has the particularity of being a fairly rare “core-collapsed” system: its stars lie increasingly closer all the way to its center. The astrophysicists chose this cluster because its proximity provides exquisite data, and its high central density makes it a favorable place for the existence of an intermediate-mass black hole.

Eduardo Vitral and Gary Mamon analyzed the velocities and positions of a few thousand stars from NGC 6397 measured with three state-of-the-art telescopes: the Hubble Space Telescope, the Gaia satellite and the Very Large Telescope in Chile (equipped with the MUSE spectrograph). The velocities of stars in a cluster trace the distribution of mass in it: the faster they move, the more mass there is. But the analysis is complex, because of our lack of perception of depth within the cluster. The two astrophysicists used a new sophisticated method to adjust to the observed motions both the way mass is distributed throughout the cluster and the shapes of the trajectories followed by stars within the cluster. Interestingly, the analysis showed that the shapes of the orbits of stars are close to random throughout the cluster, rather than the expected highly elongated orbits in the outer regions, for reasons that remain to be understood.

The models also indicate that invisible matter is required to explain the observed distributions and orbits of the stars, which could be in the form of an intermediate-mass black hole of roughly 500 the Sun's mass at the center of the cluster. However, a compact collection of unseen stars, with a spatial extent only a few percent that of the visible stars, provides by far the best match to the motions and distribution of stars on the sky. The estimated mass of these unseen stars also amounts to a few percent of that of the visible stars. This is illustrated in Figure 2 in which the observed and predicted spreads in velocities at different locations are compared.

Figure 2 Figure 2: Statistical spread of the component of velocities projected on the sky and pointing towards or away from the cluster center, for the brightest stars of NGC 6397 as a function of the projected distance from the cluster center (in logarithmic scale). Three classes of models are shown: without any invisible matter (left); with an intermediate-mass black hole of roughly 500 times the Sun's mass (middle); with an extended invisible mass, made mostly of black holes, with also white dwarfs and neutron stars (right). The red squares indicate the data points derived from the observations with the Hubble and Gaia space telescopes. The black curves show the most likely of each class of model, and the blue shaded areas highlight their uncertainties. Comparison of the three graphs shows that an extended unseen mass (right) succeeds in raising the modeled radial velocities between 0.2 and 0.4 light-years, without introducing a sharp increase at smaller distances.

The small size of the extra mass, and its lack of light suggest that it is composed of the dark remnants of massive stars, which progressively moved to the core of the globular cluster by transferring their energy to nearby less massive stars. The astrophysicists applied the theory of stellar evolution linking the final masses of stars to their masses when they formed. This led them to conclude that roughly 60% of their extended dark component ought to be stellar mass black holes, while the remaining 40% would be made of white dwarfs and neutron stars. The model suggests that there could be roughly 60 black holes with masses between 5 and 50 times the Sun's mass in the core of NGC 6397, amounting, together with the white dwarfs and neutron stars, to a total mass of 1,000 to 2,000 times that of the Sun.

This study points to a stellar “graveyard” in the core of the globular cluster, made of black holes, white dwarves, and neutrons stars. It is also the first indirect evidence of a population of black holes in a core-collapsed globular cluster. Two studies suggested in 2019 that concentrations of stellar black holes could reproduce the velocities of stars in the non-core-collapsed globular clusters Omega Cen[4] and 47 Tuc [5]. The strength of the present analysis of NGC 6397 is the estimate of both the mass and spatial extent of the unseen concentration of stars, as well as the uncertainties on these estimates (see Figure 2).

The two astrophysicists wonder whether other core-collapsed globular clusters host such concentrated “graveyards” of stars in their center, and if the locations of these clusters within the Milky Way (and their orbits around it) are different from other clusters. Existing data from the Hubble Space Telescope and improved data from the Gaia satellite could be used to unveil more of such systems. The astrophysicists also wonder whether the mergers of the tightly packed black holes within NGC 6397 may be an important source of gravitational waves, that could be detected by the LIGO experiment. When black holes merge, they receive very large impulses to balance the momentum from the anisotropic gravitational radiation . These impulses may be sufficiently large to allow the new black hole, resulting from the merger, to escape from the cluster. The result presented here is valid under the asumption that only a small fraction of the black holes are ejected from the cluster.


puce Astronomy and Astrophysics article: Vitral & Mamon, 2021, “Does NGC 6397 contain an intermediate-mass black hole or a more diffuse inner subcluster?” https://doi.org/10.1051/0004-6361/202039650 (Public version)

puce NASA press release: “Hubble uncovers a concentration of small black holes”

puce The New-York Times article: “Hunting for a Giant Black Hole, Astronomers Found a Nest of Darkness”


[1] NASA press release: “Black Hole Image Makes History; NASA Telescopes Coordinated Observations”; and
Astrophysical Journal Letters article: the Even Horizon Telescope collaboration, Akiyama et al., 2019, “First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole” https://iopscience.iop.org/article/10.3847/2041-8213/ab0ec7 (Public version)

[2] Physical Review Letters article: Abbott et al., 2020, “GW190521: A Binary Black Hole Merger with a Total Mass of 150M”  https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.125.101102 (Public version)

[3] NASA press release: “Hubble Finds Best Evidence for Elusive Mid-Sized Black Hole”; and Astrophysical Journal Letters article: Lin et al., 2020, “Multiwavelength Follow-up of the Hyperluminous Intermediate-mass Black Hole Candidate 3XMM J215022.4−055108” https://iopscience.iop.org/article/10.3847/2041-8213/ab745b (Public version)

[4] Monthly Notices of the Royal Astronomical Society article: Zocchi et al. 2018, “The effect of stellar-mass black holes on the central kinematics of ω Cen: a cautionary tale for IMBH interpretations” https://academic.oup.com/mnras/article-abstract/482/4/4713/5036534 (Public version)

[5] Astrophysical Journal article: Mann et al. 2019, “A Multimass Velocity Dispersion Model of 47 Tucanae Indicates No Evidence for an Intermediate-mass Black Hole” https://iopscience.iop.org/article/10.3847/1538-4357/ab0e6d (Public version)

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February 2021

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