Hubble photograph of globular cluster NGC6397
Astronomers have recently discovered an unexpected phenomenon at the center of globular cluster NGC 6397: a large group of small black holes hidden in the center of the cluster.
Globular clusters are very dense star systems, and these star systems are generally very old. For example, this globular cluster NGC 6397 is almost as old as the universe.
Located 7,800 light-years from Earth, this cluster is one of the closest spherical clusters to Earth. It is considered to be a core collapse cluster because of its very dense center.
Initially, astronomers believed that there was a medium-mass black hole at the center of the globular cluster. Such intermediate black holes are between supermassive black holes at the core of galaxies (up to millions of times the mass of the Sun) and stellar-mass black holes (several times the mass of the Sun) formed by the collapse of stars. Whether they exist has been a long-standing debate in astronomy. So far, only a few possible intermediate black holes have been identified.
“We found very strong evidence for an invisible mass at the center of the globular cluster. But we were surprised to find that this invisible mass is not ‘point-like’ (such as an isolated larger mass black hole), but discrete, spread over a region of a few percent of the cluster’s size.” said Eduardo Vitral of the Institute of Astrophysics (IAP) in Paris, France.
To analyze this invisible hidden mass of the cluster, the team used the stellar velocities in the cluster to determine the distribution of its total mass, that is, the mass in visible stars, faint stars, and black holes. The more mass there is in a given region, the faster the star is moving around it.
The team used previous estimates of the radial motion of the stars in the night sky to determine the true velocity of the stars within the cluster.
These precise measurements of the stars at the core of the cluster could only be made over several years with the Hubble Telescope.
Hubble Telescope data are added to calibrated observations of radial motion from the Gaia Mission, which are less accurate than those from the Hubble Telescope.
“Our analysis shows that the orbits of the stars are almost random throughout the globular cluster, rather than systematic circular or very elongated orbits.” IAP researcher Gary Mamon explained.
The team concluded that, given their mass, extent and location, the invisible mass component is likely to consist only of remnants of massive stars (such as white dwarfs, neutron stars and black holes). After gravitational interactions with nearby, less massive stars, these stellar corpses gradually sink into the center of the cluster.
This game of bouncing balls between stars is called “kinetic friction,” and by exchanging momentum, heavier stars are isolated in the core of the cluster, while lower-mass stars migrate to the periphery of the cluster.
“We use stellar evolution theory to conclude that most of the masses we find are in the form of black holes,” Mamoun said. Mamun said.
Two other recent studies have also suggested that stellar remnants, especially stellar-mass black holes, may fill the inner regions of globular clusters. Vitral added, “We are the first study to provide mass and distribution, finding that the centers of globular clusters with collapsed cores appear to be collections of small black holes.”
Astronomers also note that the finding raises the possibility that the merging of black holes in these tightly packed globular clusters could be an important source of gravitational waves. Gravitational waves are fluctuations in spacetime itself that can be observed by gravitational wave observatories.
The new study is published in the February 11, 2021 issue of the journal Astronomy & Astrophysics.
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