Astronomers found something they didn̵
Globular clusters are very dense star systems with their stars closely packed together. These systems are usually very old-the globular cluster NGC 6397 that this study focuses on is almost as old as the universe itself. This star cluster is located 7,800 light-years from the Earth, making it one of the closest spherical star clusters to the Earth. Because of their very dense nuclei, they are called core collapse clusters.
At first, astronomers thought that there was a medium-mass black hole in the globular cluster. These are the long-sought “missing links” between the supermassive black holes located in the core of the galaxy (millions of times the mass of our sun) and the stellar mass black holes formed after the collapse (several times the mass of our sun). The stars and their existence are heatedly debated. So far, only a few candidates have been identified.
“We found very strong evidence that there is an invisible mass in the dense core of a globular cluster, but we were surprised to find that this extra mass is not’pointy’ (isolated massive black holes can be expected), but It expands by a few percent of the size of the astrological cluster,” said Eduardo Vitral of the Institute of Astrophysics (IAP) in Paris, France.
In order to detect the elusive hidden mass, Vitral and Gary Mamon, also at IAP, use the velocities of stars in the cluster to determine the distribution of their total mass, that is, visible stars, dark stars, and mass holes in black. The greater the mass in a certain location, the faster the star will move around it.
Researchers used previous estimates of the tiny inherent motion of stars (their apparent motion in the sky) to determine the true velocity of stars within the cluster. These precise measurements of the stars in the core of the cluster can only be observed for several years with the Hubble Telescope. Hubble data is added to well-calibrated and proper motion measurements provided by the European Space Agency’s Gaia Space Observatory, which are less accurate than Hubble observations in the core.
“Our analysis shows that in the entire globular cluster, the orbits of the stars are almost random, rather than systematically circular or very elongated,” Mamon explained. These moderately elongated track shapes limit the internal mass.
Astronomers looking for the medium-mass black hole at the center of the globular cluster NGC 6397 discovered something they didn’t expect: a concentration of smaller black holes instead of a huge black hole lurking there. credit: NASA Goddard Space Flight Center
The researchers concluded that, given their mass, range, and location, the invisible components can only consist of the remnants of massive stars (white dwarfs, neutron stars, and black holes). These stellar corpses gradually sink into the center of the cluster after gravitationally interacting with nearby smaller stars. This stellar pinball game is called “dynamic friction”. Through momentum exchange, heavier stars are isolated in the core of the cluster, while low-mass stars migrate to the periphery of the cluster.
“Our conclusion using stellar evolution theory is that most of the extra mass we find is in the form of black holes,” Mamon said. Two other recent studies have also suggested that stellar remnants, especially black holes of stellar mass, may fill the inner regions of globular clusters. Vitral added: “We are the first study to provide quality and extent. The study appears to be a collection of black holes in the center of a globular cluster with a collapsed core.”
Astronomers also noticed that this discovery increases the possibility of merging these closely packed black holes into globular clusters, which may be an important reason for this phenomenon. Gravitational waves, Rippling in time and space. This phenomenon can be detected by the laser interferometer gravity wave observatory experiment, which was funded by the National Science Foundation and operated by the California Institute of Technology in Pasadena, California. with In Cambridge, Massachusetts.
For more information on this research, please read Unexpected Discovery: The Hubble Space Telescope has discovered the concentration of small black holes.
Reference: “Does NGC 6397 contain intermediate mass black holes or more diffuse internal subclusters?” Written by Eduardo Vitral and Gary A. Mamon on February 11, 2021, Astronomy and Astrophysics.
DOI: 10.1051 / 0004-6361 / 202039650
This Hubble Space Telescope It is an international cooperation project between NASA and ESA (European Space Agency). The NASA Goddard Space Flight Center in Greenbelt, Maryland manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland is responsible for the scientific operations of the Hubble Telescope. STScI is operated for NASA by the Association of Universities for Astronomical Research in Washington, DC