Astronomers using the NASA/ESA Hubble Space Telescope have found evidence for dozens of stellar-mass black holes lurking in the core-collapsed, globular cluster NGC 6397, one of the closest globular clusters to Earth.
An artist’s impression of a group of stellar-mass black holes in the center of NGC 6397. Image credit: NASA / ESA / Hubble / N. Bartmann.
Also known as GCl 74, the cluster is 13.4 billion years old and so formed not long after the Big Bang.
When Institut d’Astrophysique de Paris astronomers Dr. Eduardo Vitral and Dr. Gary Mamon set out to study NGC 6397, they expected to detect imprints of intermediate-mass black holes in the cluster’s core.
To look for these extremely rare objects they analyzed the positions and velocities of the cluster’s stars.
They used previous estimates of the stars’ proper motions from Hubble images of the cluster and proper motions provided by ESA’s Gaia space observatory.
Knowing the distance to NGC 6397 allowed the astronomers to translate the proper motions of these stars into velocities.
“Our analysis indicated that the orbits of the stars are close to random throughout the globular cluster, rather than systematically circular or very elongated,” Dr. Mamon said.
“We found very strong evidence for invisible mass in the dense central regions of the cluster, but we were surprised to find that this extra mass is not point-like but extended to a few percent of the size of the cluster,” Dr. Vitral added.
This image shows the nearby globular cluster NGC 6397, located at a distance of 7,800 light-years in the southern constellation Ara. Image credit: NASA / ESA / T. Brown & S. Casertano, STScI / J. Anderson, STScI.
This invisible component could only be made up of the remnants (white dwarfs, neutron stars, and black holes) of massive stars whose inner regions collapsed under their own gravity once their nuclear fuel was exhausted.
The stars progressively sank to the cluster’s center after gravitational interactions with nearby less massive stars, leading to the small extent of the invisible mass concentration.
Using the theory of stellar evolution, the researchers concluded that the bulk of the unseen concentration is made of stellar-mass black holes, rather than white dwarfs or neutron stars that are too faint to observe.
“Our study is the first finding to provide both the mass and the extent of what appears to be a collection of mostly black holes in a core-collapsed globular cluster,” Dr. Vitral said.
“Our analysis would not have been possible without having both the Hubble data to constrain the inner regions of the cluster and the Gaia data to constrain the orbital shapes of the outer stars, which in turn indirectly constrain the velocities of foreground and background stars in the inner regions,” Dr. Mamon said.
The findings were published in the journal Astronomy & Astrophysics.
Eduardo Vitral & Gary A. Mamon. 2021. Does NGC 6397 contain an intermediate-mass black hole or a more diffuse inner subcluster? A&A 646, A63; doi: 10.1051/0004-6361/202039650