Research box title
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 offered an intermediate mass of black hole (IMBH). These IMBHs are the long-sought “missing link” between supermassive black holes (many millions of times our solar mass) that lie at the core of galaxies, and black mass stars (several times our solar mass) that follow the collapse of a single massive star. Their mere existence is hotly debated. Only a few candidates have been identified to date.
“We found very strong evidence for an invisible mass in the dense core of the spherical group, but we were surprised to find that this extra mass is not ‘point-like’ (which would be expected for a solid massive black hole). ), but expanded to a few percent of the size of the group, ‘says 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 presented by the European Space Agency’s Gaia space station, but which are 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.
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 lip is called ‘dynamic friction’, where heavier stars are separated in a group’s core and stars with a lower mass migrate to the circumference 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,” Mamon said. Two other recent studies have also suggested that stellar remnants, particularly 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 added.
The astronomers also note that this discovery raises the possibility that fusion of these densely packed black holes in spherical clusters could be an important source of gravitational waves, ripples during space-time. Such phenomena can be detected by the LIGO (Laser Interferometer Gravitational-Wave Observatory) experiment. LIGO is funded by the National Science Foundation and is managed by Caltech and MIT.
The Hubble Space Telescope is a project of international collaboration 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 Hubble scientific operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, DC