Hubble Uncovers Concentration of Small Black Holes

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).

Hubble's view of dazzling globular cluster NGC 6397
Hubble’s view of dazzling globular cluster NGC 6397 Credit: ESA/Hubble Information Centre

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 ( [1], for example).

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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 provided by ESA’s Gaia space observatory, 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.

Callout of the Black Hole Concentration in NGC 6397 (Artist's Impression)
Callout of the Black Hole Concentration in NGC 6397 (Artist’s Impression) Credit: ESA/Hubble Information Centre

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

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“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 cluster,” added Vitral.

Artist’s Impression of the Black Hole Concentration in NGC 6397
Artist’s Impression of the Black Hole Concentration in NGC 6397 Credit: ESA/Hubble Information Centre

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.

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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.

Ground-based Image of Globular Cluster NGC 6397
Ground-based Image of Globular Cluster NGC 6397 Credit: ESA/Hubble Information Centre

“Our study is the 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,” 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 indirectly constrain the velocities of foreground and background stars in the inner regions,” added Mamon, attesting to an exemplary international collaboration.

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Wide-Field View of Globular Cluster NGC 6397 Credit: ESA/Hubble Information Centre

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 important source of gravitational waves recently detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) experiment.

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[1] In 2020, new data from the NASA/ESA Hubble Space Telescope provided the strongest evidence to date for a mid-sized black hole.

[2] Proper motion describes how fast objects move in the sky.

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[3] The Hubble data for this study were provided by A. Bellini, who measured the proper motions for over 1.3 million stars in 22 globular clusters, including NGC 6397.

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

The international team of astronomers in this study consists of E. Vitral and G. A. Mamon. The results have been published in Astronomy & Astrophysics.


ESA/Hubble Information Centre

Journal Reference

Does NGC 6397 contain an intermediate-mass black hole or a more diffuse inner subcluster?


We analyze proper motions from the Hubble Space Telescope (HST) and the second Gaia data release along with line-of-sight velocities from the MUSE spectrograph to detect imprints of an intermediate-mass black hole (IMBH) in the center of the nearby, core-collapsed, globular cluster NGC 6397.

For this, we use the new MAMPOSST-PM Bayesian mass-modeling code, along with updated estimates of the surface density profile of NGC 6397. We consider different priors on velocity anisotropy and on the size of the central mass, and we also separate the stars into components of different mean mass to allow for mass segregation.

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The velocity ellipsoid is very isotropic throughout the cluster, as expected in post-core collapsed clusters subject to as strong a Galactic tidal field as NGC 6397. There is strong evidence for a central dark component of 0.8 to 2% of the total mass of the cluster.

However, we find robust evidence disfavoring a central IMBH in NGC 6397, preferring instead a diffuse dark inner subcluster of unresolved objects with a total mass of 1000 to 2000 M⊙, half of which is concentrated within 6 arcsec (2% of the stellar effective radius).

These results require the combination of HST and Gaia data: HST for the inner diagnostics and Gaia for the outer surface density and velocity anisotropy profiles. The small effective radius of the diffuse dark component suggests that it is composed of compact stars (white dwarfs and neutron stars) and stellar-mass black holes, whose inner locations are caused by dynamical friction given their high progenitor masses.

We show that stellar-mass black holes should dominate the mass of this diffuse dark component, unless more than 25% escape from the cluster. Their mergers in the cores of core-collapsed globular clusters could be an important source of the gravitational wave events detected by LIGO.

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