From NASA/ESA Hubble Telescope: “Hubble Uncovers Concentration of Small Black Holes”

NASA/ESA Hubble Telescope

From NASA/ESA Hubble Telescope

February 11, 2021

Ray Villard
Space Telescope Science Institute, Baltimore, Maryland

Bethany Downer
ESA/Hubble Space Telescope

Science Contacts:
Eduardo Vitral
Institut d’Astrophysique de Paris (IAP)(FR)

Dr. Gary A. Mamon
Institut d’Astrophysique de Paris (IAP) (FR)

Compass Image for NGC 6397
Credits: Image: NASA/ESA, T. Brown, S. Casertano, and J. Anderson (STScI) (US)
Science: NASA/ ESA E. Vitral and G. Mamon (Institut d’Astrophysique de Paris (IAP) (FR)


The idea that black holes come in different sizes may sound a little odd at first. After all, a black hole by definition is an object that has collapsed under gravity to an infinite density, making it smaller than the period at the end of this sentence. But the amount of mass a black hole can pack away varies widely from less than twice the mass of our Sun to over a billion times our Sun’s mass. Midway between are intermediate-mass black holes (IMBHs) weighing roughly hundreds to tens of thousands of solar masses. So, black holes come small, medium, and large.

However, the IMBHs have been elusive. They are predicted to hide out in the centers of globular star clusters, beehive-shaped swarms of as many as a million stars. Hubble researchers went hunting for an IMBH in the nearby globular cluster NGC 6397 and came up with a surprise. Because a black hole cannot be seen, they carefully studied the motion of stars inside the cluster, that would be gravitationally affected by the black hole’s gravitational tug. The amplitudes and shapes of the stellar orbits led to the conclusion that there is not just one hefty black hole, but a swarm of smaller black holes – a mini-cluster in the core of the globular.

Why are the black holes hanging out together? A gravitational pinball game takes place inside globular clusters where more massive objects sink to the center by exchanging momentum with smaller stars, that then migrate to the cluster’s periphery. The central black holes may eventually merge, sending ripples across space as gravitational waves.


Astronomers found something they weren’t expecting at the heart of the globular cluster NGC 6397: a concentration of smaller black holes lurking there instead of one massive black hole.

Globular clusters are extremely dense stellar systems, which host stars that are closely packed together. These systems are also typically very old — the globular cluster at the focus of this study, NGC 6397, is almost as old as the universe itself. This cluster resides 7,800 light-years away, making it one of the closest globular clusters to Earth. Due to its very dense nucleus, it is known as a core-collapsed cluster.

At first, astronomers thought the globular cluster hosted an intermediate-mass black hole (IMBH). These IMBHs are the long-sought “missing link” between supermassive black holes (many millions of times our Sun’s mass) that lie at the cores of galaxies, and stellar-mass black holes (a few times our Sun’s mass) that form following the collapse of a single massive star. Their mere existence is hotly debated. Only a few candidates have been identified to date.

“We found very strong evidence for an invisible mass in the dense core of the globular cluster, but we were surprised to find that this extra mass is not ‘point-like’ (that would be expected for a solitary massive black hole) but extended to a few percent of the size of the cluster,” said Eduardo Vitral of the Paris Institute of Astrophysics, (IAP) (FR).

To detect the elusive hidden mass, Vitral and Gary Mamon, also of IAP, used the velocities of stars in the cluster to determine the distribution of its total mass, that is the mass in the visible stars, as well as in faint stars and black holes. The more mass at some location, the faster the stars travel around it.

The researchers used previous estimates of the stars’ tiny proper motions (their apparent motions on the sky), which allow for determining their true velocities within the cluster. These precise measurements for stars in the cluster’s core could only be made with Hubble over several years of observation. The Hubble data were added to well-calibrated proper motion measurements provided by the European Space Agency’s Gaia space observatory, but which are less precise than Hubble’s observations in the core.

ESA (EU)/GAIA satellite .

“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. These moderate-elongation orbital shapes constrain what the inner mass must be.

The researchers conclude that the invisible component can only be made of the remnants of massive stars (white dwarfs, neutron stars, and black holes) given its mass, extent and location. These stellar corpses progressively sank to the cluster’s center after gravitational interactions with nearby less massive stars. This game of stellar pinball is called “dynamical friction,” where, through an exchange of momentum, heavier stars are segregated in the cluster’s core and lower-mass stars migrate to the cluster’s periphery.

“We used the theory of stellar evolution to conclude that most of the extra mass we found was in the form of black holes,” said Mamon. Two other recent studies had also proposed that stellar remnants, in particular, stellar-mass black holes, could populate the inner regions of globular clusters. “Ours is the first study to provide both the mass and the extent of what appears to be a collection of mostly black holes in the center of a core-collapsed globular cluster,” added Vitral [Astronomy & Astrophysics].

The astronomers also note that this discovery raises the possibility that mergers of these tightly packed black holes in globular clusters may be an important source of gravitational waves, ripples through spacetime. Such phenomena could be detected by the LIGO (Laser Interferometer Gravitational-Wave Observatory) experiment. LIGO is funded by the National Science Foundation and operated by Caltech and MIT.

MIT /Caltech Advanced aLigo .

See the full article here.


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Wide Field Camera 3 [WFC3]


Advanced Camera for Surveys [ACS]

NASA Hubble Advanced Camera for Surveys.

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NASA Hubble Cosmic Origins Spectrograph.

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA’s Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute (STScI), is a free-standing science center, located on the campus of The Johns Hopkins University and operated by the Association of Universities for Research in Astronomy (AURA) for NASA, conducts Hubble science operations.

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