This is an artist’s impression created to visualize the concentration of black holes in the center of NGC 6397. In fact, the small black holes here are too small to directly observe the capacity of any existing or planned future telescope, including Hubble. This nuclear-collapsed spherical group is predicted to house more than 20 black holes. Credit: ESA / Hubble, N. Bartmann
Astronomers have found something they did not expect, in the heart of the spherical cluster NGC 6397: a concentration of smaller black holes hiding there instead of one massive black hole.
Global clusters are extremely dense galaxies that offer stars that are closely packed together. These systems are usually very old – the spherical group in the focus of this study, NGC 6397, is almost as old as the universe itself. This cluster is 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 a core-collapsed cluster.
Initially, astronomers thought that the spherical group contained a black hole between the masses. This is the long-sought “missing link” between supermassive black holes (many millions of times our sun’s mass) that lie in the stars of the galaxies, and black masses (several times our sun’s mass) that arise after the collapse. of a single massive star. Their mere existence is hotly debated. Only a few candidates have been identified to date.
The amount of mass that a black hole can pack away varies greatly from less than twice the mass of our sun to more than a billion times the mass of our sun. In the middle there are black holes in intermediate masses that weigh about hundreds to tens of thousands of solar masses. So, black holes are small, medium and large. Credit: NASA, ESA, T. Brown, S. Casertano and J. Anderson (STScI)
‘We found very strong evidence for an invisible mass in the dense core of the spherical group, but we were surprised to see that this extra mass is not’ point-like ‘(which would be expected for a solid massive black hole). ), but has expanded. to a few percent of the size of the group, ”said Eduardo Vitral of the Paris Institute of Astrophysics (IAP) in Paris, France.
To detect the elusive hidden mass, Vitral and Gary Mamon, also from IAP, used the velocities of stars in the group to distribute its total mass, that is, the mass in the visible stars, as well as in faint stars. and determine black. gate. The more mass in one place or another, the faster the stars move around it.
The researchers used previous estimates of the small motions of the stars (their apparent motions to the sky), which determine the true velocity in the group. These exact measurements for stars in the core of the group could only be done with Hubble over a few years of observation. The Hubble data were added to well-calibrated real motion measurements provided by the European Space Agency’s Gaia space observation, which is less accurate than Hubble’s observations at the core.
“Our analysis indicated that the orbits of the stars are close to random throughout the spherical group, rather than systematically circular or very elongated,” Mamon explained. These orbital shapes with moderate elongation limit the inner mass.
Astronomers looking for a medium-sized black hole in the heart of the spherical cluster NGC 6397, found something they did not expect: a concentration of smaller black holes hiding there instead of one massive black hole. Credit: NASA Goddard Space Flight Center
The researchers conclude that the invisible component can only be made from the remains of massive stars (white dwarfs, neutron stars and black holes) given their mass, extent and location. These stellar bodies gradually sank to the center of the cluster after gravitational interaction with less massive stars in the environment. This game of star flip is called ‘dynamic friction’, where heavier stars are separated in a core of the cluster and stars with a lower mass migrate to the periphery of the cluster.
“We used the theory of stellar evolution to conclude that the extra mass we found was in the form of black holes,” Mamon said. Two other recent studies have also suggested that stellar remnants, especially black masses, may populate the inner regions of spherical clusters. “Ours is the first study to offer both the mass and extent of a collection of mostly black holes in the center of a nucleated spherical group,” Vitral said.
The astronomers also note that this discovery raises the possibility that fusion of these densely packed black holes in spherical clusters is an important source of gravity waves, wrinkles through space-time. Such phenomena can be detected by the Laser Interferometer Gravitational-Wave Observatory experiment, which is funded by the National Science Foundation and operated by Caltech in Pasadena, California and MIT in Cambridge, Massachusetts.
Read Unexpected discovery: Hubble Space Telescope uncovered the concentration of small black holes for more information on this research.
Reference: “Does NGC 6397 contain a black hole with intermediate mass or a diffuser inner subcluster?” by Eduardo Vitral and Gary A. Mamon, February 11, 2021, Astronomy and astrophysics.
DOI: 10.1051 / 0004-6361 / 202039650
The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, controls the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts scientific operations of Hubble. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, DC