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This is an artist’s impression created to visualize the concentration of black holes at the center of NGC 6397. In reality, the small black holes here are far too small for the direct viewing capabilities of any existing or future telescope. planned, including Hubble. It is predicted that this collapsed globular cluster could host more than 20 black holes. Credit: ESA / Hubble, N. Bartmann
Astronomers have found something they didn’t expect at the heart of globular cluster NGC 6397: a concentration of tiny black holes lurking there instead of a massive one. black hole.
Globular clusters are extremely dense star systems that host tightly packed stars. These systems are also generally very old – the globular cluster at the center 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 core, it is known as the collapsed core cluster.
At first, astronomers thought the globular cluster was home to a black hole of intermediate mass. They are the “missing link” sought for a long time between the supermassive black holes (several million times the mass of our Sun) which are at the heart of the galaxies, and the black holes of stellar mass (sometimes the mass of our Sun). which form as a result of the collapse of a single massive star. Their very existence is hotly debated. Only a few candidates have been identified to date.
The amount of mass a black hole can take away varies considerably from less than twice the mass of our Sun to more than a billion times the mass of our Sun. Halfway there are intermediate-mass black holes weighing about hundreds to tens of thousands of solar masses. Thus, black holes are small, medium and large. Credit: NASA, ESA, T. Brown, S. Casertano and J. Anderson (STScI)
“We found very strong evidence of an invisible mass in the dense core of the globular cluster, but we were surprised to find that this additional mass is not ‘point’ (which would be expected for a black hole massive solitary) but extended to a few percent of the size of the cluster, ”said Eduardo Vitral of the Institut d’Astrophysique de Paris (IAP) in Paris, France.
To detect the elusive hidden mass, Vitral and Gary Mamon, also from IAP, used the velocities of the stars in the cluster to determine the distribution of its total mass, that is, the mass in visible stars. , as well as in faint and dark stars. Holes. The more mass there is in one place, the faster the stars move around.
The researchers used previous estimates of the stars’ proper tiny motions (their apparent motions across the sky), which help determine their actual speeds within the cluster. These precise measurements of the stars at the heart of the cluster could only be made with Hubble over several years of observation. The data from Hubble was added to appropriate well-calibrated motion measurements provided by the European Space Agency’s Gaia Space Observatory, which are less accurate than Hubble’s observations in the nucleus.
“Our analysis indicated that the orbits of the stars are almost random throughout the globular cluster, rather than systematically circular or very elongated,” says Mamon. These moderately elongated orbital shapes constrain what the internal mass should be.
Astronomers looking for an intermediate mass black hole in the heart of the NGC 6397 globular cluster have found something they did not expect: a concentration of tiny black holes hidden there instead of a black hole massive. Credit: NASA Goddard Space Flight Center
The researchers conclude that the invisible component can only consist of the remnants of massive stars (white dwarfs, neutron stars and black holes) given its mass, extent and location. These stellar corpses gradually sank towards the center of the cluster after gravitational interactions with less massive stars nearby. This stellar pinball game is called “dynamic friction,” where, through an exchange of momentum, heavier stars are separated in the core of the cluster and stars of lower mass migrate to the periphery of the cluster.
“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, especially stellar mass black holes, could populate the internal regions of globular clusters. “Ours is the first study to provide both the mass and extent of what appears to be a collection of mostly black holes at the center of a collapsed globular cluster,” added Vitral.
Astronomers also note that this discovery raises the possibility that the mergers of these tightly packed black holes into globular clusters could be an important source of gravitational waves, undulates in space-time. Such phenomena could be detected by the Laser Interferometer Gravitational Wave Observatory experiment, funded by the National Science Foundation and operated by Caltech in Pasadena, California. WITH in Cambridge, Massachusetts.
To learn more about this research, read Unexpected Discovery: Hubble Space Telescope Discovers Concentration of Small Black Holes.
Reference: “Does NGC 6397 contain an intermediate mass black hole or a more diffuse internal sub-cluster?” by Eduardo Vitral and Gary A. Mamon, February 11, 2021, Astronomy and astrophysics.
DOI: 10.1051 / 0004-6361 / 202039650
The The Hubble Space Telescope is an international cooperation project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland operates the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts the science operations of Hubble. STScI is operated for NASA by the Association of Universities for Astronomical Research in Washington, DC
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