About 7,800 light-years away — within our galactic neighbourhood — is globular cluster NGC 6397, basically a wad of stars held together by gravity. That bunch of stars was previously thought to have an intermediate-sized black hole at its centre. But upon further inspection, a team from the Paris Institute of Astrophysics has determined that the cluster actually hosts a pod of smaller black holes, holding things together in a more diffuse system.
Previous research had suggested that the cluster’s core could be populated with such a conglomeration of star-sized black holes, but this paper goes a step further by also measuring the mass and extent of those objects. The team’s research was published this week in the journal Astronomy and Astrophysics.
To identify the mystery at the centre of the cluster, the researchers looked at how stars near its centre were moving using data from two space telescopes, the Hubble Space Telescope and the European Space Agency’s Gaia observatory.
“We found very strong evidence for an invisible mass in the dense core of the globular cluster, but we were surprised to find that this extra mass is not ‘point-like,’” said Eduardo Vitral of the Paris Institute of Astrophysics, in a NASA press release. If just one black hole were responsible, the mystery mass would be more densely concentrated. “Ours is the first study to provide both the mass and the extent of what appears to be a collection of mostly black holes in the centre of a core-collapsed globular cluster.”
Intermediate black holes have long been considered a missing link in black hole evolution. Perhaps less in the public eye than supermassive black holes (whose existence is proven) or teeny-weeny primordial black holes (which remain theoretical), intermediate black holes, as their name suggests, would help astronomers understand how these enigmatic structures develop.
“Our analysis indicated that the orbits of the stars are close to random throughout the globular cluster, rather than systematically circular or very elongated,” explained Gary Mamon, also at the Paris Institute of Astrophysics, in the same release. If the stars’ orbits were coordinated, it would suggest one governing massive object. Instead, the stars seemed to be invested in their own ad-hoc movements. Mamon and Vitral believe this is because dense stellar remnants like white dwarves, neutron stars, and black holes — formed when stars reach the end of their lifespan and collapse in on themselves — sank toward the centre of the cluster, in a sort of three-dimensional Plinko. Conversely, stars of lower mass made their way to the cluster’s periphery.
“The authors have carried out a thorough analysis, and the conclusions of this work present an unexpected development in the hunt for [intermediate black holes] in globular clusters,” Misty Bentz, an astrophysicist at Georgia State University who is unaffiliated with the recent paper, said in an email. “However, there are a lot of assumptions that are necessary when carrying out studies like this, and the results still leave room for the possibility of an [intermediate black hole] in this globular cluster.
Artistic illustrations of the globular cluster, shown in the video above, look like someone perforated space-time with a stellar-scale shotgun. Chasms of immense gravitational force loom out of the cluster; the remnants of dead stars holding their living brethren together in a gravitational lattice (or ensnaring them, depending on your outlook). It’s not a surprise that such this region would contain so many white dwarves, neutron stars, and black holes; NGC 6397 is an extremely ancient cluster, clocking in at 12.6 billion years old, give or take half a billion, giving stars plenty of time to complete their life cycles.
Bentz said that the new study doesn’t fully show that the cluster contains many small black holes rather than one larger one, “but if that is the case, then it makes the origin of supermassive black holes even more mysterious than they already are!” That’s because, she explained, “we expect supermassive black holes must have grown from smaller seeds. But the result from this study would suggest that it is actually hard to merge lots of small black holes in a globular cluster, because the globular cluster is old and yet the small black holes are still hanging around individually, not merged together.”
It’s an intriguing finding. Though black holes were predicted to exist back in 1916 by Albert Einstein, it was only two years ago, in April 2019, that scientists captured an actual image of one. Clearly, we have a lot more to learn about these mysterious objects.