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Black holes come in three sizes: stellar size is anything that can be formed by a star going supernova; supermassive black holes are millions, if not billions, of times the mass of the Sun and are located in the cores of galaxies; and intermediate is any black hole in between.
Since they don’t emit light, all black holes are very elusive, but intermediate-mass black holes are particularly so. We only have a few possible candidates. Astronomers thought they had found one at the center of the globular cluster NGC 6397. New observations have revealed something even more peculiar at its core: a large group of smaller black holes.
As reported in Astronomy & Astrophysics, the motion of the stars in the globular cluster suggested the presence of a massive, dense, and yet invisible “something” at the center. This is usually an indication that a black hole might be lurking there. An estimated mass of 1,000 and 2,000 times larger than the Sun would make it an intermediate-mass black hole if it was a single object. And that ‘if’ is crucial.
The researchers studied the motion in the globular cluster in detail and it appears this massive concentration at its core didn’t come from a single source but from a larger distribution. This suggests the presence of multiple black holes moving about in the center of the cluster.
"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," co-author Eduardo Vitral of the Paris Institute of Astrophysics (IAP), said in a statement.
Key to the structures of a globular cluster is a particular process called dynamical friction. Over eons, the gravitational interactions between the celestial bodies in the globular cluster separates the more massive objects (pushing them towards the center) from the lighter ones (which are thrown towards the edges. It’s this game of stellar pinball, as the authors call it, that created the congregation of black holes at its center.
The work was possible thanks to the combined observations of the Hubble Space Telescope and the European Space Agency’s Gaia observatory. Hubble has previously measured the distance of NGC 6397 at 7,800 light-years from Earth with incredible precision.
The new work provides insights into the motion and orbits of the stars, which in turn allowed the researchers to establish the mass of the black hole group and the fact that it wasn’t a single object.
"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 co-author Gary Mamon also from the IAP.
The two researchers believe that in the future it is possible for these black holes to merge, emitting gravitational waves that could be spotted by observatories on Earth.
