A dense mass of stars a few thousand light-years away hides a surprise at its core. Instead of a relatively thick black hole, astronomers have discovered that globular star cluster NGC 6397 is wrapped around a cluster of smaller clusters.
Not only could this help us better understand the formation of larger black holes, it also suggests that globular clusters could be of great importance to gravitational wave astronomy, as the black holes will inevitably come closer together in the direction of collision.
Globular clusters are often considered “fossils” of the early Universe. They are very dense, globular clusters of about 100,000 to 1 million very old stars, some – like NGC 6397 – nearly as old as the Universe itself. In each spherical cluster, all its stars are formed simultaneously 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 the galaxies in which they orbit. Lately, however, astronomers have taken a closer look at them as possible homes of an elusive class of objects – medium black holes.
As the name suggests, these middleweights are located between stellar mass and supermassive black holes, the latter of which are usually found in the centers of galaxies.
While the boundaries between medium mass black holes and supermassive black holes are currently not very well defined, average mass black holes are generally considered to be larger than a typical collapsed star (up to a hundred solar masses), but not supermassive (between million and a billion times more mass than a typical stellar black hole).
However, solid evidence for the existence of medium mass black holes is sparse and largely unclear. Theory and models suggest they can be found in globular clusters, the gravitational core around which the stars gather, as larger galaxies around supermassive black holes.
The properties of NGC 6397, about 7,800 light-years away, suggested there could be one of these middleweights in the center.
Since we can’t see black holes (because they don’t emit detectable radiation), astronomers have taken a closer look at the orbits of stars in the cluster, based on years of Hubble data, to see if they are an intermediate mass of black hole.
“We found very strong evidence for 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 to find that this extra mass is not point-like. ‘is.’ (that would be expected for a lone huge black hole) but expanded to a few percent of the size of the cluster. ‘
This corresponds to a type of resistance known as dynamic friction, where objects in the cluster exchange momentum, sending denser, heavier objects to the core and less massive objects to the edge.
Dead stars such as white dwarfs, neutron stars and black holes are denser than stars in the main sequence, so they move inward and send the lighter stars out.
“We used the theory of the evolution of stars 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 papers, which found that instead of medium-mass black holes, stellar-mass black hole populations could inhabit the central regions of globular clusters. Now those findings have been validated.
“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,” said Vitral.
This is useful information for both the study of black holes of stellar mass and the hunt for black holes of medium mass. Now that we have observational evidence that this can happen, astronomers can refine their searches to rule out globular clusters that behave in the same way.
There are also implications for other research on black holes.
As the objects will continue to sink towards the center of the cluster, the team believes they will eventually spiral into each other and fuse. Ultimately – over a very, very long time – this could result in a black hole of average mass.
More immediately, this ongoing process suggests that the nuclei of such clusters could be very important to gravitational wave astronomy. Because they are so densely packed, the processes should be speeded up, meaning we could look at these regions to study both pre-fusion conditions and try to anticipate the gravitational wave events that will occur when the black holes getting together.
The research is published in Astronomy and Astrophysics