A dense group of stars a few thousand light-years away harbors a surprise in its core. Rather than a relatively thick black hole, astronomers have discovered that the globular group NGC 6397 is wrapped around a group of smaller, stellar masses.
Not only could this help us better understand the formation of larger black holes, but it suggests that globular clusters may be of great interest for gravitational wave astronomy, as black holes inevitably approach collision.
Globular star clusters are often considered “fossils” of the early universe. They are very dense spherical clusters, about 100,000 to 1 million very old stars, some – like NGC 6397 – almost as old as the Universe itself. In any globular group, all its stars formed at the same time, from the same gas cloud. The Milky Way has about 150 known globular groups.
These objects are excellent tools for studying, for example, the history of the universe or the dark matter content of the galaxies it orbits. Recently, however, astronomers have looked more closely at them as potential houses of an evasive class of objects – black holes with intermediate masses.
As the name suggests, these medium weights are placed between black stellar and supermassive holes, the latter usually being found in the center of galaxies.
While the boundaries between intermediate black holes and supermassive black holes are currently not very well defined, intermediate black holes are generally considered to be larger than a typical collapsed star (up to one hundred solar masses), but not supermassive (between one million and one billion times more mass than a typical stellar black hole).
Firm evidence for the existence of intermediate-mass black holes is, however, rare and largely inconclusive. Theory and modeling suggest that they could be found in globular clusters, the gravitational core around which stars gather, like larger galaxies around supermassive black holes.
The properties of NGC 6397, about 7,800 light-years away, suggest that there may be one of these average weights at its center.
Since we cannot see black holes (because they do not emit any detectable radiation), astronomers looked more closely at the orbits of the stars in the group, based on data from the Hubble years, to see if they indicated a black hole in the mass.
“We found very strong evidence for an invisible mass in the dense core of the globular group,” said astronomer Eduardo Vitral of the Paris Institute of Astrophysics, “but we were surprised to find that this additional mass is not ‘point-like.'” This would be expected for a solitary massive black hole), but extended to a few percent of the cluster size. “
This is consistent with a type of traction known as dynamic friction, in which objects in the cluster change momentum, sending denser and more massive objects to the core and less massive objects to the periphery.
Dead stars, such as white dwarfs, neutron stars, and black holes, are denser than main-sequence stars, so they move inward, sending the stars lighter outward.
“We used the theory of stellar evolution to conclude that most of the extra mass we found was in the form of black holes,” said astronomer Gary Mamon of the Institute of Astrophysics in Paris.
It is also consistent with two recent studies, which found that instead of intermediate-mass black holes, populations of stellar-mass black holes could live in the central regions of globular clusters. These findings have now been validated.
“Ours is the first study to provide both the mass and magnitude of what appears to be a collection of black holes in most of the center of a collapsed globular cluster,” said Vitral.
This is useful information both in the study for black holes with stellar mass and for searching for black holes with intermediate mass. Now that we have observational evidence that this can happen, astronomers can refine their searches to exclude globular groups that behave the same way.
There are implications for other black hole research as well.
As the objects continue to sink toward the center of the cluster, the team believes that they will eventually begin to spiral into each other and unite. Finally – very, very long now – this could lead to a black hole with an intermediate mass.
More immediately, this ongoing process suggests that the nuclei of such groups could be very important for gravitational wave astronomy. Because they are so tightly packed, processes should be accelerated, which means we could look at these regions both to study pre-fusion conditions and to try to prevent gravitational wave events that will occur when black holes fuse.
The research was published in Astronomy and astrophysics.