a cluster of black holes at the heart of a globular cluster

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When examining the nucleus of a globular cluster “close” to Earth, astronomers expected to stumble upon an elusive object: a black hole of intermediate mass. Eventually, they came across a concentration of small, stellar-mass black holes. Details of the study are published in the journal Astronomy and astrophysics.

The globular clusters are concentrations of hundreds of thousands of stars confined in a sphere varying in size from twenty to a few hundred light years. All its stars were formed at the same time, from the same cloud of gas. There are currently about 150 of these clusters in the Milky Way, but there could be more. Finally, these objects are often considered as “fossils” insofar as most of them formed very early in the Universe.

Intermediate mass black hole

Found around 7,800 light years of the earth, NGC 6397 is one of those clusters. It is also one of the closest to Earth. Recently, Eduardo Vitral and Gary A. Mamon, of the Institut d’Astrophysique de Paris, took an interest in him to probe the possible presence of a very rare object inside: a intermediate mass black hole.

As their name suggests, these “average weights” are located between black holes of stellar mass (up to a hundred solar masses) and supermassives (several million, even billions of stellar masses), found in the center of galaxies.

According to the authors, these “intermediate” objects could be found in the gravitational nucleus of globular clusters, around which the stars rally. The properties of NGC 6397 also suggested that there might be one of these “middle weights” at its center.

NGC image 6397 photographed between July 2004 and June 2005 by Hubble. Credits: NASA, ESA / T. Brown and S. Casertano (STScI)

A multitude of small black holes

To probe the possible presence of such an object, the researchers relied on images of the cluster captured by Hubble over several years. They also combined this data with that collected by ESA’s Gaia satellite, which examines the positions, distances and movements of stars. Thanks to all this information, the researchers were able to estimate the mass distribution inside this cluster.

We have found very strong evidence of an invisible mass in the dense nucleus of the globular cluster“, Confirmed astronomer Eduardo Vitral. “On the other hand, we were surprised to note that this additional mass was not contained in a very precise point, but distributed over a larger area (a few percentages of the size of the cluster)“.

We know that “dead” stars, such as white dwarfs, neutron stars, and black holes, are denser than main sequence stars. As a result, they move inward in these clusters, sending less dense stars outward.

Here, the researchers’ calculations showed that this invisible mass distributed in the center of the cluster was not a black hole of intermediate mass, nor a collection of white dwarfs or neutron stars, but indeed a concentration of stellar mass black holes.

Our study is the first to provide both the mass and extent of what appears to be a collection of mostly black holes in the center of a collapsed globular cluster.“, Emphasizes Eduardo Vitral.

Future mergers?

This work will be useful for the future since we now know that several black holes of stellar mass can be found confined to the center of globular clusters. Starting from this principle, astronomers can refine their research by excluding clusters that behave in the same way.

We could also imagine that all these little black holes could wrap around each other. to eventually merge. As a result, they could therefore give rise to a black hole of intermediate mass. In the meantime, the processes leading to these mergers could also generate gravitational waves that can be picked up by our instruments.

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