A dense starburst a few thousand light-years away has been a cozy surprise in its core. Instead of a relatively large black hole, astronomers have discovered that the globular cluster NGC 6397 surrounds itself around a small cluster of stellar masses.
Not only could this help us better understand the formation of larger black holes, but it suggests that globular clusters could be of great interest for gravitational wave astronomy, as black holes inevitably approach the collision.
Star globular clusters are often considered “fossils” of the early universe. They are very dense spherical clusters of about 100,000 to a million very old stars, some, like NGC 6397, almost as old as the Universe itself. In any globular cluster, all its stars formed at the same time, from the same gas cloud. The Milky Way has about 150 known globular clusters.
These objects are excellent tools for studying, for example, the history of the Universe or the dark matter content of orbiting galaxies. Recently, however, astronomers have looked at them more closely as the possible houses of an elusive class of objects: the black holes of intermediate mass.
As its name suggests, these average weights lie between black holes of stellar mass and supermass, the latter of which are usually found in the centers of galaxies.
Although the boundaries between intermediate-mass black holes and supermassive black holes are currently not very well defined, intermediate-mass black holes are generally considered larger than a typical collapsed star (up to a hundred solar masses). ) but not supermassive (between a million and a billion times more massive than a typical stellar black hole).
However, strong evidence for the existence of intermediate-mass black holes is scarce 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, suggested that there could be one of these weights in the center.
Because we can’t see black holes (because they don’t emit any detectable radiation), astronomers looked more closely at the orbits of the stars in the cluster, based on years of Hubble data, to see if they indicated an intermediate element. massive black hole.
“We found very strong evidence of an invisible mass in the dense core of the globular cluster,” said astronomer Eduardo Vitral of the Paris Institute of Astrophysics in France, “but we were surprised that this additional mass is not.” punctual (this was expected for a solitary massive black hole) but extended to a few percent of the size of the cluster “.
This is consistent with a type of drag known as dynamic friction, in which objects in the cluster exchange momentum, sending denser, massive objects toward the core and less massive objects toward the outskirts.
Dead stars like white dwarfs, neutron stars, and black holes are denser than main-sequence stars, so they move inward and cause lighter stars to come out.
“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 Paris Institute of Astrophysics.
It is also consistent with two recent studies, which found that instead of intermediate-mass black holes, populations of stellar-mass black holes could inhabit the central regions of globular clusters. Now these findings have been validated.
“Ours is the first study to provide both the mass and the extent of what appears to be a collection of black holes mostly in the center of a collapsed globular cluster,” Vitral said.
This information is useful both in the study of stellar mass black holes and in the search for intermediate mass black holes. Now that we have observational evidence that this can happen, astronomers can refine their searches to rule out globular clusters that behave the same way.
There are also implications for other research on the black hole.
As the objects will continue to sink toward the center of the cluster, the team believes they will eventually begin to spiral and merge. Eventually, in a very, very long time, this could result in a black hole of intermediate mass.
More immediately, this ongoing process suggests that the nuclei of these clusters could be very important for gravitational wave astronomy. Because they are so packaged, the processes have to be accelerated, which means we could look at these regions both to study the pre-fusion conditions and to try to avoid the gravitational wave events that will occur when the gravitational waves merge. black holes.
The research has been published in Astronomy and astrophysics.