A dense cluster of stars a few thousand light-years away offers a surprise at its core. Instead of one relatively thick black hole, astronomers found that the spherical cluster NGC 6397 revolved around a group of smaller star masses.
Not only can this help us better understand the formation of larger black holes, but it suggests that spherical clusters may be of great importance for gravity wave astronomy, as the black holes can inevitably move closer together.
Global star clusters are often considered ‘fossils’ of the early Universe. These are very dense, spherical clusters of about 100,000 to 1 million very old stars, some – like NGC 6397 – almost as old as the universe itself. In any spherical group, all its stars formed simultaneously from the same cloud gas. The galaxy has about 150 known spherical clusters.
These objects are excellent tools for studying, for example, the history of the universe, or the contents of the dark matter of the galaxies orbiting them. Recently, however, astronomers have taken a closer look at them as the potential homes of an elusive class of objects – black holes between the masses.
As the name suggests, these middle weights sit between stellar mass and supermassive black holes, the latter of which usually occur in the centers of galaxies.
While the boundaries between mass with black mass and supermassive black holes are currently not very well defined, black holes between mass mass are generally considered larger than a typical collapsing star (up to a hundred solar masses), but not as supermassive (between ‘ a million and a billion times more mass than a typical star black hole).
However, solid evidence for the existence of black mass with intermediate mass is scarce and largely unconvincing. Theory and modeling suggest that they can be found in spherical clusters, the gravity of which the stars converge, such as larger galaxies around supermassive black holes.
The properties of NGC 6397, about 7,800 light-years away, suggested that one of these center weights would be in its center.
Since we can not see black holes (because they emit no perceptible radiation), astronomers looked at the orbits of stars in the group more closely, based on years of Hubble data, to see if they give an interim indication – mass black hole.
“We found very strong evidence for an invisible mass in the dense core of the spherical group,” said astronomer Eduardo Vitral of the Paris Institute of Astrophysics in France, “but we were surprised to discover that this extra mass does not ‘punctual’ is not. ‘(it would be expected for a solid, massive black hole), but it extends to a few percent of the size of the group.’
This corresponds to a type of resistance known as dynamic friction, in which objects in the group exchange momentum, sending denser, more massive objects to the core and less massive objects to the outskirts.
Dead stars such as white dwarfs, neutron stars and black holes are denser than main sequence stars, so they move inward and send out the lighter stars.
“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.
This is also consistent with two recent articles, which found that, instead of black holes between intermediate masses, populations of black masses in stellar masses could inhabit the central regions of spherical clusters. Now the findings have been confirmed.
“We are the first study to offer both the mass and extent of a collection of mostly black holes in the center of a nucleated spherical group,” Vitral said.
This is useful information, both in the study of black masses of star masses and the hunt for black mats with intermediate masses. Now that we have observational evidence that this could happen, astronomers can refine their searches to eliminate spherical clusters that behave in the same way.
There are also implications for other research on black holes.
Because the objects will continue to sink in the middle of the group, the team believes that they will eventually begin to turn into each other and merge. Eventually – over a very, very long time from now – it could result in a black hole between the masses.
More constantly, this ongoing process suggests that the core of such clusters may be very important for gravity wave astronomy. Because they are so closely packed, the processes need to be accelerated, which means we can look at these regions to study the conditions before the merger, and to try to predict the gravitational wave events that will occur when the black holes merge.
The research was published in Astronomy & Astrophysics.