Research box title
Astronomers found something they hadn’t expected at the heart of globular cluster NGC 6397: a concentration of smaller black holes lurking there instead of one huge black hole.
Globular clusters are extremely dense stellar systems, which harbor stars that are densely packed together. These systems are also typically very old – the globular star cluster on which this study focuses, NGC 6397, is almost as old as the universe itself. This cluster is located 7,800 light-years away, making it one of the closest globular star clusters on Earth. Due to its very dense core, it is known as a core collapsed cluster.
Initially, astronomers believed that the globular cluster was home to a mid-mass black hole (IMBH). These IMBHs are the long-sought ‘missing link’ between supermassive black holes (many millions of times the mass of our sun) that lie in the cores of galaxies and black holes of stellar mass (a few times the mass of our sun) that are form according to the collapse of a single massive star. Only their 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 globular cluster, but we were surprised to find that this extra mass is not ‘point-like’ (which would be expected for a solitary massive black hole) but expanded into a few percent of the cluster size, ”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 cluster to determine the distribution of their total mass, that is, the mass in the visible stars, but also in faint stars and black. holes. The more mass in a given location, the faster the stars travel around it.
The researchers used previous estimates of the stars’ small actual movements (their apparent movements in the sky), which make it possible to determine their actual speeds within the cluster. These accurate measurements for stars in the cluster’s core could only be made with Hubble over several years of observation. The Hubble data was added to well-calibrated eigenmotion measurements provided by the European Space Agency’s Gaia space observatory, but which are less accurate than Hubble’s observations at the core.
“Our analysis indicated that the orbits of the stars throughout the cluster are almost random, rather than systematically circular or very elongated,” Mamon explains. These orbital shapes with moderate elongation determine what the inner mass should be.
The researchers conclude that the invisible component can only be made from the remnants of massive stars (white dwarfs, neutron stars and black holes) given its mass, size and location. These stellar corpses gradually sank towards the center of the cluster after gravitational interactions with nearby less massive stars. This stellar pinball game is called “dynamic friction”, where an exchange of momentum separates heavier stars at 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,” Mamon said. Two other recent studies had also suggested that stellar remnants, especially black holes with a stellar mass, could populate the innermost regions of globular clusters. “Our study is the first to provide both the mass and size of what appears to be a collection of mostly black holes at the center of a globular cluster collapsed into the core,” added Vitral.
The astronomers also note that this discovery opens the possibility that mergers of these tightly packed black holes in globular clusters could be a major source of gravitational waves, ripples through spacetime. Such phenomena could be detected by the LIGO (Laser Interferometer Gravitational-Wave Observatory) experiment. LIGO is funded by the National Science Foundation and 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 operates the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland conducts Hubble science operations. STScI is administered for NASA by the Association of Universities for Research in Astronomy in Washington, DC